CHAPTER VII. Remarks on the Distribution of Marine and Fresh-Water Crustacea

REMARKS ON THE DISTRIBUTION OF MARINE AND FRESH-WATER CRUSTACEA

=A. Marine.=

The great majority of the Crustacea are inhabitants of the sea. From a Zoogeographical point of view we divide the sea into three chief regions, each of which is characterised by a special kind of fauna—the littoral, the pelagic, and the abyssal regions.

The =littoral= region, which comprises all the shallow coastal waters down to about 100 fathoms, varies very greatly in its physical character according to the nature of the coast, its geological constitution, latitude, etc., but, on the whole, it is characterised by variability of temperature and salinity, by the presence of sunlight, and by the continuous motion of its waves. On the shores of the large oceans this region is also greatly affected by the tides. It is inhabited by a vast assemblage of Crustacea, all of which are dependent upon a solid substratum, either of rock or sand, or of vegetable or animal growth, upon which they may wander in search of food, or in which they may hide themselves. In consequence, the character of the Crustacea on any shore is largely determined by its geological nature.

Although a certain number of Entomostraca such as Copepoda (Harpacticidae and Cyclopidae), Ostracoda (Cypridae and Cytheridae), and a few Operculata are littoral in habit, it is the Malacostraca, from their larger size and variety of form, which give the character to coastal waters.

On rocky coasts, especially those affected by tides, a great many kinds of Shore-crab are found, which hide at low tide in the rock-pools and under stones. Carcinus maenas is characteristic of the rocky coasts of the North Sea, while it is replaced in warmer seas and all round the tropics by Crabs of the family Grapsidae, which are typical rock-livers, and exceedingly agile in clambering over tide-washed rocks. Porcellanidae are also very common under stones at low tide on rocky beaches. Such typical Shore-crabs as these are remarkably resistant to desiccation, and can live out of water for an astonishing time; nor do they require a change of water provided they have access to the air. The edible crab (Cancer pagurus) and the lobsters (Homarus and Palinurus) are dependent on rocks, but they rarely come close in-shore, preferring depths of a few fathoms.

Sandy coasts are preferred by Shrimps and Prawns, which haunt the shallow coastal waters in shoals; and in the sand are found all the Crabs whose respiratory mechanism is specially adapted for life in these regions, e.g. Hippidea or Mole-crabs, Corystes, Matuta, Calappa, etc.

Characteristic of sandy bottoms are also the Thalassinidea, such as Callianassa, which excavate galleries in the sand. On tropical sandy shores various species of Ocypoda and Gelasimus are conspicuous, which have deserted the sea, and live in burrows which they excavate on the shore. Gelasimus is especially abundant in the muddy sand of tropical mangrove swamps.

Besides the rocky and sandy coasts we must distinguish the muddy shores and bottoms which support a large amount of vegetable and animal growth. These, besides harbouring the greater number of Amphipods and Isopods, are also the natural home of the Dromiacea and Oxyrhyncha, or Spider-crabs, among which the habit is common of decking themselves out with pieces of weed or animal growth in order to harmonise better with their surroundings. Pagurids are also especially abundant in the deeper waters of these coasts.

Coral-reefs support a characteristic Crustacean fauna. In the growing coral at the reef-edge a number of small Cyclometopa are found, e.g. Chlorodius, Actaea, Xantho, which are finely sculptured and often coloured so as to harmonise with the coral. Alpheidae also, Shrimp-like Macrura with highly asymmetrical claws, which can emit a sharp cracking sound with the larger claw, are commonly found in pools on the reef. In the coralshingle formed by abrasion from the reef-edge at a few fathoms depth, Leucosiidae are found, in which, again, respiratory mechanisms for filtering sand from the gills are present.

Besides the geological nature of the coast, latitude has a very important bearing upon the distribution of littoral Crustacea. Indeed, the present distribution of littoral Crustacea appears to be far more determined by the temperature of the coastal waters than by the presence of any land-barriers, however formidable. We may distinguish an Arctic, Antarctic, and Circumtropical zone.

The =Arctic= zone includes the true Arctic seas, and stretches right down through boreal regions towards the sub-tropical seas. Almost all the truly Arctic forms penetrate fairly far south, the Arctic seas being characterised more by the absence of temperate forms than by the presence of forms peculiar to itself. At the same time it must be noted that the individuals from the coldest regions often grow to an enormous size, a characteristic which is physiologically unexplained.

A great many of the Crustacea characteristic of this region are circumpolar, i.e. they are not restricted in range to either the Atlantic or Pacific. This is especially true of the extremely northern types, e.g. Crangonidae and Hippolytidae, but it is also true of a number of Crustacea which do not now occur as far north as Greenland or Bering Strait, so that there is no longer any free communication for them between Pacific and Atlantic. This gives rise to a discontinuous distribution in the two oceans, exemplified in the common Shrimp, Crangon vulgaris, which is found on the temperate European coasts and on the Pacific coasts of Japan and Eastern America. The same is true of Eupagurus pubescens and E. bernhardus.

At the same time the boreal Atlantic and Pacific have their peculiar forms. Thus the European and American Lobsters are confined to the Atlantic, while the North Pacific possesses a very rich array of Lithodinae, which cannot be paralleled in the Atlantic.

We may explain the community of many littoral forms to both the North Atlantic and Pacific coasts by the continuous coast-line uniting them, which in former times possibly did not lie so far north, or else was not subjected to so rigorous a climate as now.

In the =Antarctic= zone we are presented with very different relations, since the great continents are drawn out to points towards the south, and are isolated by vast tracts of intervening deep sea. Nevertheless, certain littoral forms are circumpolar, e.g. the Palinurid Iasus and the Crabs Cyclograpsus and Hymenosoma. The genus Dromidia is common to Australia and South Africa, though it is apparently absent from South America.

The Isopod genus Serolis is confined to Antarctic seas. The majority are littoral species, and they are distributed round the coasts of Patagonia, Australia, and Kerguelen in a manner that certainly suggests a closer connection between these shores in the past. These facts are, on the whole, evidence in favour of the former existence of an Antarctic continent stretching farther north and connecting Australia, Africa, and S. America—a supposition that has been put forward to account for the distribution of the Penguins, Struthious birds, Oligochaets, Crayfishes, etc., in these regions (see pp. 215–217).

In considering the Arctic and Antarctic faunas the supposed phenomenon of bipolarity must be mentioned, i.e. the occurrence of particular species in Arctic and Antarctic seas, but not in the intermediate regions. This discontinuous type of distribution was upheld for a variety of marine animals by Pfeffer, Murray, and others, but it has been very adversely criticised by Ortmann. As far as the Arctic and Antarctic Decapod fauna in general are concerned, the north polar forms are quite distinct from the south polar. Typical of the former are Hippolyte, Sclerocrangon, Hyas, Homarus, etc.; of the latter, Hymenosoma, Dromidia, Iasus. It appears, however, that in certain special cases, bipolarity of distribution may be produced owing to the operation of peculiar causes. Two such cases seem to be fairly well established. Crangon antarcticus occurs at the two poles, and apparently not in the intermediate regions; but, as Ortmann points out, it is represented right down the West American coast by a very closely related form, C. franciscorum. The waters on the tropical western coasts both of Africa and America are exceedingly cool, and it appears that in this way the Crangon may have migrated across the tropical belt, leaving a slightly modified race to represent it in this intermediate region. The other case of bipolarity is afforded by the “Schizopod,” Boreomysis scyphops, which occurs at both poles, but is not known from the tropics. This is a pelagic species, and we know that the Mysidae often descend to considerable depths. We also know that the Mysidae are dependent on cold water, only occurring in boreal or temperate waters. We may safely suppose, therefore, that the migration of this species has taken place by their forsaking the surface-waters as the tropics were approached, and passing down into the depths where the temperature is constantly low even in the tropics.

The dependence of Crustacea upon the temperature of the water is also illustrated by the distribution of the Lithodinae. The headquarters of this family are in the boreal Pacific, with a few scattered representatives in the boreal Atlantic. The cool currents on the western coasts of America, however, have permitted certain forms to migrate as far south as Patagonia, where they still have a littoral habit. In the tropical Indo-Pacific, where a few species occur, they are only found in deep waters. Thus at these various latitudes, by following cool currents or migrating into deep water, they are always subjected to similar conditions of temperature. The same kind of thing is observed in Arctic seas, where deep-sea forms are apt to take on secondarily a littoral habit owing to the temperature of the depths and of the shore being the same.

Despite the impassable barriers of land which now sever the tropical oceans, we can yet speak of a =circumtropical= zone possessing many species common to its most widely separated parts. Such circumtropical species, occurring on both the Atlantic and Pacific coasts of tropical America, on the West African coast, and in the Indo-Pacific, are various Grapsidae, Calappa granulata and its allies, and certain Albunea. The most striking instance of all is that of the Land-crabs. Of Ocypoda, the greater number of species occur in the Indo-Pacific, but representatives are also found on the tropical Eastern and Western American coasts and on the West African coast, and the same is true of Gelasimus. The genus Cardisoma, belonging to a different group of Land-crabs, is also typically circumtropical.

For this community of the circumtropical species we may certainly advance in explanation the comparatively recent formation of the Isthmus of Panama. Besides the resemblance of the Crustacea on the east and west coasts of the isthmus, we have an actual identity of species in several cases, e.g. Pachycheles panamensis and Hippa emerita, and the same thing has been observed for the marine fish.

Another connexion, at any rate during early tertiary times, which probably existed between now isolated tropical coasts, was across the Atlantic from the West Indies to the Mediterranean and West African coasts. Numerous facts speak for this connexion. Species of Palinurus and Dromia occur in the West Indies and the Mediterranean, which only differ from one another in detail, and a connexion between these two regions has been urged from the minute resemblances of the late Cretaceous Corals of the West Indies with those of the Gosau beds of S. Europe, and also of the Miocene land-molluscs of S. Europe with those at the present time found in the West Indies.

To account, then, for the present distribution of littoral Crustacea we must imagine that great changes have taken place during comparatively recent times in the coast-lines of the ocean, but the guiding principle in both the past and present has been temperature, and this factor enables us, despite the immense changes in the configuration of the globe that must have taken place, to divide the coasts latitudinally into Arctic, Antarctic, and Circumtropical zones.

=Pelagic Crustacea= belong chiefly to the Copepoda (Calanidae, Centropagidae, Candacidae, Pontellidae, Corycaeidae), a few Ostracoda (Halocypridae and Cypridinae), and among Malacostraca a few Amphipoda (Hyperina), some “Schizopoda,” and among Decapoda only the Sergestidae, if we except the few special forms which live on the floating weeds of the Sargasso Sea, e.g. the Prawns Virbius acuminatus and Latreutes ensiferus, and the Brachyura Neptunus sayi and Planes minutus. Besides these Crustacea which are pelagic as adults, there is an enormous host of larval forms, both among Entomostraca and Malacostraca, which are taken in the surface-plankton.

In dealing with the Copepoda we have already mentioned the vast pelagic shoals of these organisms which occur at particular times of the year, and have an important influence on fishing industries. Anomalocera pattersoni (Fig. 27, p. 60) is a good instance of this. It is a large Heterarthrandrian, about 3 mm. long, with the body of a fine bluish green colour; it has a remarkable power of springing out of the water, so that a shoal has the appearance of fine rain upon the surface of the sea. It occurs in the open Atlantic and Mediterranean, but comes into the coasts during violent storms; the Norwegian fishermen hail its presence in the fjords as the sign of the approach of the summer herring.

It was Haeckel who first clearly distinguished between “neritic” plankton, the species of which have their centres of distribution in shallow coastal waters and die out gradually as the open ocean is approached, and “oceanic” plankton which is habitually found in the open sea, and though it may invade the coasts is not dependent on the sea-bottom in any way. It appears that although these two kinds of plankton may get mixed up by currents and storms, they are always recruited by new generations from the neritic or oceanic stations proper to each kind.

Common oceanic species, found chiefly in the open Atlantic and in the North Sea, are Anomalocera pattersoni, Calanus finmarchicus, Centropages typicus, Metridia lucens, Oithona plumifera, etc. Common neritic species in the Channel and other coastal waters are Centropages hamatus, Euterpe acutifrons, Oithona nana, Temora longicornis, etc. It was found by Gough that although the true oceanic species invade the Channel from the open Atlantic to the west, they become rarer and rarer as they advance up the Channel. Thus the plankton midway between the Lizard and Ushant at all times of year is about 70 per cent. oceanic, while at the line drawn from Portland to the Cap de la Hague it is about 35 per cent. Seasonal changes in the salinity of the Channel water, chiefly due to the influx of oceanic water from the Atlantic, as observed by Matthews, do not clearly influence the distribution of oceanic and neritic forms. The influx of highly saline water from the Atlantic was most marked during the winter months up to February. From February to May the highly saline water receded, and during the summer months at the line drawn between Portland and the Cap de la Hague the salinity was rather low. This was increased in November by a patch of oceanic water being cut off from the main mass and passing up Channel, and it is noteworthy that during this month the highest percentage of oceanic forms was taken in the plankton of this region.

Calanus finmarchicus affords a clear instance of the way in which the plankton may be carried about for great distances by means of currents. This species has its home in the subarctic seas, but is carried down in the spring by the East Icelandic Polar stream to its spawning-place south of Iceland; the enormous shoals produced here are carried back, continually multiplying, along the coasts of Norway during the summer and autumn.

Besides these great migrations, the plankton organisms perform daily movements, the majority of the Crustacea avoiding the surface during the day, and often going down to as much as seventy fathoms or more, and only coming up to the surface at night. Others, however, e.g. Calanus finmarchicus, behave in the converse manner, preferring the sunlit surface to swim in.

Owing to their dispersal by means of oceanic currents the pelagic Crustacea do not offer any very striking features in regard to their distribution, and the possibility of always finding congenial temperatures by passing into the upper or under strata of water enables them to live in almost all seas. The tropical species of Sergestidae are mostly circumtropical, i.e. unhindered by the present barriers of land.

The =Abyssal= regions of the sea contain many of the most interesting Crustacea. Families entirely confined to the abyss are the Eryonidae, Pylochelidae, and certain Caridean Prawns (Psalidopodidae, etc.), but there are a great number of normally littoral genera which have representatives in deep water. If we draw the limit between the littoral and abyssal regions at about 200 metres, we can characterise the latter as absolutely dark except for the presence of phosphorescent organisms, with the temperature at a little above zero, and with a comparative lack of dissolved oxygen in the water. These conditions bring about remarkable modifications in the structure and life-histories of the inhabitants of the deep sea; we have already touched on the modifications of the visual organs and on the presence of phosphorescence in many of the animals; other points to be noticed are the usually uniform yellowish or bright red coloration, the frequent delicacy of the tissues without much calcification, variations in the structure of the breathing organs, e.g. in Bathynomus giganteus and Encephaloides armstrongi, and the loss of the larval development. Owing to the similarity of conditions in the deep sea all over the globe most of its inhabitants are universally distributed. It is also a striking fact that species are found in the deep sea of the tropics whose nearest allies occur, not in the littoral seas of the tropics, but in those of the temperate region. This fact has already been alluded to in dealing with the distribution of the Lithodinae. Alcock remarks that between 50–500 fathoms in the Indian Ocean are found Crabs such as Maia, Latreillia, and Homola, regarded as characteristic of the north temperate seas; the lobster Nephrops andamanica, taken at 150–400 fathoms, is closely allied to the Norwegian N. norwegica; and nine species of “Schizopoda,” which are certainly temperate forms, occur in the Indian Ocean at depths of 500–1750 fathoms.

=B. Fresh-Water.=

If we except the Crayfishes and River-crabs, the Crustacean fauna of running water is exceedingly poor, but in all standing fresh-water, from the smallest pond to the large lakes and inland seas, Crustacea, especially Entomostraca, are abundant and characteristic, and form an important item in the food of fresh-water fishes. In small ponds a vast assemblage of Cladocera is met with; these animals multiply with great rapidity by parthenogenesis, especially during spring and summer, but on the advent of untoward conditions sexual individuals are produced, which lay fertilised winter-eggs which lie dormant until favourable conditions again arise. As Weismann first pointed out, the frequency with which sexual individuals are produced in the various species is closely correlated with the liability of the water in which they live to dry up; so that the Cladocera which inhabit small ponds usually have at least two “epidemics” of sexual individuals, one during early summer and the other before the onset of winter.

Besides Cladocera, the Phyllopoda (e.g. Apus, Artemia, etc.) inhabit small pools; and also a great number of Cyclopidae. Of the other fresh-water families of Copepoda, viz. Centropagidae and Harpacticidae, inhabitants of small pieces of water are Diaptomus castor, as opposed to the other species of Diaptomus which are pelagic, and a number of Harpacticidae (Canthocamptus), the members of this family living in the weed or mud of either small ponds or else on the shores of the larger lakes. The greater number of Ostracoda are found in similar situations.

A district like the Broads of Norfolk, which consists partly of slowly-moving streams and partly of extensive stretches of shallow water, supports a Crustacean fauna intermediate in character between that found in small ponds and the truly pelagic fauna characteristic of deep lakes. A very complete list of the Crustacea of the Norfolk Broads, with an interesting commentary on their distribution, is given by Mr. Robert Gurney. We miss here the pelagic Cladocera, such as Leptodora, Bythotrephes, Holopedium, etc., which form so characteristic a feature of large lakes; at the same time, besides a rich development of the Cladocera, Cyclopidae, and Harpacticidae, which haunt the weeds and mud of shallow waters, we find such species as Polyphemus pediculus and Bosmina longirostris among Cladocera, which are otherwise confined to large bodies of water, and a few pelagic Diaptomus, e.g. D. gracilis. The fauna is also complicated in this district by the proximity to the sea and the frequently high salinity of the water, which allows a number of typically marine Copepods to pass up the estuaries and intermingle with typically fresh-water species; such are Eurytemora affinis among the Centropagidae, and several species of Harpacticidae (see p. 62).

The large lakes of the world, such as the continental lakes of Europe and America, or of our own Lake District, reproduce on a small scale the varied conditions which appertain to the ocean—as in the ocean, we can recognise in these lakes a littoral, a pelagic, and an abyssal region. Our knowledge of the physiography of lakes is largely due to the classical work of Forel, and the following account of the physical conditions in the various regions is condensed from his book.

The =littoral= region is sharply marked off from the others by the relative instability of its physical conditions, owing to the agitation of its waters, the affluence of streams and drainage, and the constant changes of temperature. The water in this region generally contains a good deal of solid matter in suspension, while the shelving banks of the lake support a wealth of vegetable growth, both of Algae and of Phanerogams, down to about 20–25 metres. At this depth the daylight does not penetrate sufficiently to admit of the growth of green plants, so that this region marks the limit, both physical and biological, between the littoral and the abyssal zones. In this littoral region there flourish a great quantity of Entomostraca, most of which are also found in small ponds where similar conditions of life prevail—the pelagic species only penetrating rarely, and by accident, into its waters. At the beginning of July Mr. H. O. S. Gibson and myself found that the weedy littoral region of Grasmere contained almost entirely large quantities of the Cladoceran Eurycercus lamellatus, and a number of Cyclops fuscus and C. strenuus. In the littoral zone of large lakes, Amphipods, Isopods, and fresh-water shrimps may also be met with, but this applies more to the lakes of the Tropics and of the Southern Hemisphere.

The =pelagic= region is distinguished from the littoral by the greater purity and transparency of its waters, and by the relative stability of the temperature, the annual range, even at the surface, in Geneva being from 4°–20° C., while at 100 metres the water has a uniform temperature of 4° or 5° C. The upper strata are, of course, brightly illuminated, but at 20 metres there is hardly sufficient light for green plants to grow, and at 100 metres it is completely dark. The inhabitants of this region, known collectively as plankton, spend their whole life swimming freely in the water, sometimes at the surface and sometimes in the deeper strata. They consist chiefly of Diatoms, Protozoa, Rotifera, and Crustacea. The pelagic Crustacea, especially the Cladocera, are often the most curiously and delicately built creatures. Leptodora hyalina, which is quite transparent, is the largest of them, attaining to three-quarters of an inch in length, though Bythotrephes longimanus is nearly as large if we include the immense spine which terminates the body. Holopedium gibberum, which is the commonest of all in Grasmere lake, but not so frequently met with in the other English lakes, is peculiar in that its body is enveloped in a spherical mass of transparent jelly, sometimes a quarter of an inch in diameter, so that the contents of a tow-net jar full of Holopedium have something of the consistency of boiled sago. The enormous quantities in which these animals often occur during summer is very astonishing; but to be truly appreciated tow-nettings should be taken at the surface of the lake either during night-time when there is not much moonlight, or else on a dark still day when there is a quiet drizzle falling on the surface of the water. In bright sunshine the plankton passes below the surface into the lower strata, and can be usually taken by sinking the tow-net some 10–20 feet, or to even greater depths in the water. The exact reason of these periodic migrations out of the light, and their dependence on other physical conditions, such as temperature and the agitation of the water, is not clearly understood. It appears, however, that when the water is rough, plankton always passes into the deeper regions. Besides the species mentioned, the minute Bosminidae, whose trunked heads are suggestive under the microscope of elephants, and Polyphemus pediculus are among the commonest pelagic Cladocera, though neither Polyphemus nor Bythotrephes ever form shoals. The above-mentioned genera are characteristic of the larger lakes in the Northern Hemisphere. Our knowledge of the Crustacean plankton of tropical lakes and of those of the Southern Hemisphere is limited (but see p. 216).

A very important constituent of lake-plankton is furnished by the Copepoda, especially of the genus Diaptomus. With the exception of Holopedium, by far the commonest Crustacean in Grasmere during July was found by Mr. Gibson and myself to be D. caeruleus.

At the same time a number of Cyclopidae, e.g. Cyclops strenuus, may occur in the pelagic region in considerable quantities, though they were never found by us in such numbers as Diaptomus.

The life-cycle of the pelagic Entomostraca has been studied in both the Cladocera and the Copepoda. In some of the Cladocera Weismann at first supposed that males had altogether disappeared, and that reproduction was entirely parthenogenetic. It appears, however, that all the pelagic species have at least one sexual period, namely, in the autumn, when resting eggs are produced which lie dormant during the winter. The pelagic Copepods may either produce resting eggs for the winter (Diaptomus), or else the winter is passed through in the Nauplius stage, the larvae hibernating in the mud until the spring (Cyclopidae).

We have so far only dealt with fresh-water Entomostraca. There are, in addition, a number of Malacostraca which inhabit fresh water, and some of these are found in the abyssal region of the great lakes, which must now be considered.

The physical conditions of the =abyssal= region are still more stable than those of the pelagic region, since the water is never disturbed, the bottom is always composed of a fine mud, the temperature is constant at 4°–5° C., and there is a total absence of light. It was hardly expected that animals would inhabit this region, until Forel discovered Asellus aquaticus in a depth of forty metres in the Lake of Geneva, and subsequently showed that quite a number of animals, including a Hydra, several worms, Molluscs, Crustacea, and larval Insects, may be found in these or even much greater depths.

The Crustacea of the abyssal region are two in number, and these have been found in a number of European lakes; Niphargus puteanus, a blind Amphipod closely allied to Gammarus; and Asellus forelii, allied to A. aquaticus and A. cavaticus, which may be either quite blind or else retain the rudiments of eyes.

These two Crustacea, under a practically identical form, are also found in the subterranean waters of Europe, and Forel considers that they have arrived in the abysses of the lakes from the subterranean channels, and are not derivatives of the littoral fauna.

Having completed our short review of lacustrine Crustacea, we may deal with the =subterranean and cave Crustacea=, which, as far as light and temperature are concerned, are subjected to very similar conditions to those dwelling at the bottom of deep lakes. The inhabitants of the subterranean waters have been chiefly brought to light in Artesian wells, etc., while the cavedwellers have been investigated especially at Carniola and in the American caves.

A number of species of Cyclopidae and Cypridae, many of which are blind and colourless, have been brought up in well-water. The Amphipod Niphargus puteanus has long been known from a similar source in England and all over Europe, and several other blind Gammarids inhabit the subterranean waters and caves in various parts of the world. Among Isopods, Asellus cavaticus is recorded from wells and caves in various parts of Europe, Caecidotea stygia and C. nickajackensis from the Mammoth and Nickajack Caves in America, and two very remarkable blind Isopods are described by Chilton from the subterranean waters of New Zealand, viz. Cruregens fontanus, whose nearest allies are the marine Anthuridae, and the Isopods Phreatoicus typicus and P. assimilis, which bear an extraordinary resemblance superficially to Amphipods. Besides these, a small number of subterranean Decapoda are known which retain the eye-stalks but are without functional ommatidia. These are Troglocaris schmidtii, in Hungary, related to the fresh-water Atyid Xiphocaris of East Indian and East Asiatic fresh waters rather than to the South European Atyephyra; Palaemonetes antrorum, from artesian wells in Texas; and several species of Cambarus from the Eastern United States. A blind species of Cambarus, C. stygius, has been described from the caves of Carniola, and if this determination is correct, is the sole Cambarus occurring outside America.

It will be seen from the above account that the subterranean Crustacea are an exceedingly interesting and, in many respects, archaic group, many of which have survived in these isolated and probably uncompetitive districts, while many secular changes were going on in the quick world overhead.

The remaining fresh water =Malacostraca= may be mentioned under the headings of the groups to which they belong.

Only one “Schizopod,” apart from Paranaspides, is known from fresh-water lakes, viz. Mysis relicta, which was discovered in 1861 by Lovén in the Scandinavian lakes, and has since been found in the Finnish lakes, the Caspian Sea, Lake Michigan, and other localities in N. America, and Lough Erne in Ireland. This species is closely related to Mysis oculata of Greenlandic seas.

In the Southern Hemisphere we have a species of Anaspides, A. tasmaniae, occurring in mountain streams and tarns in Tasmania, a related form which haunts the littoral zone of the Great Lake in Tasmania, and a small species, Koonunga cursor, occurs in a little stream near Melbourne.

Of the Isopoda certain genera, viz. Asellus and Monolistra, are confined to fresh water, others, such as Sphaeroma, Idothea, Alitropus, and Cymothoa, have occasional fresh-water representatives. Packard describes a remarkable blind Isopod, Caecidotea, from the Mammoth Cave of Kentucky, which occupies a very isolated position, and in the same work gives a very complete exposition of the cave-fauna of North America and Europe.

The Phreatoicidae are a curious family of Isopods confined to the fresh waters of Australia and New Zealand, which bear a remarkable resemblance to Amphipods, being laterally compressed and possessing a subchelate hand on the anterior thoracic leg. These Isopods are exceedingly common in small mountain pools and in streams in Tasmania, and in the Great Lake in that country I have recently found a number of species which, together with some species of Amphipods, make up the dominant feature in the Crustacean fauna. One of these species may grow to fully an inch in length. The family is confined to the temperate regions, and is usually found on mountains. A number of species are known from the mainland of Australia, one coming from a high elevation on Mount Kosciusko, and another (Phreatoicopsis) from the forests of Gippsland attaining a great size, and living among damp leaves, etc.

The fresh-water Amphipoda all belong to the families Talitridae, Gammaridae, and Haustoriidae (see p. 137).

Among the Talitridae, or Sand-hoppers, Orchestia and Talitrus have marine as well as fresh-water and land representatives, while the American Hyalella is entirely from fresh water, most of the species being peculiar to Lake Titicaca. Many of these animals are partly emancipated from an aquatic life. Thus Orchestia gammarellus, which is common on the sea-shore of the Mediterranean, frequently penetrates far inland, and was found in large numbers by Kotschy near a spring 4000 feet up on Mount Olympus.

Talitrus sylvaticus is very common among fallen leaves and decaying timber in Tasmania and Southern Australia, many miles from the sea, and often at an elevation of several thousand feet.

Among the Gammaridae, certain genera, e.g. Macrohectopus (Constantia), from Lake Baikal, are purely fresh-water. An enormous development of Gammaridae was discovered by Dybowsky in Lake Baikal, comprising 116 species, and lately a number more have been found by Korotneff. The majority of these were originally placed in the genus Gammarus, but Stebbing has rightly created a number of peculiar genera for them. Certain species are, however, placed in more widely distributed genera, e.g. Gammarus and Carinogammarus, which is also represented in the Caspian Sea. Korotneff found some remarkable transparent pelagic forms (Constantia) swimming in the abyssal regions at about 600 metres depth, the majority of them being blind, but some possessing rudimentary eyes, often on one side only.

Besides various species of Gammarus, a number of other Gammaridae are frequently found in brackish water. Among Haustoriidae Pontoporeia has representatives in both the oceans and inland lakes of the northern hemispheres (see p. 137).

Of the Decapoda, seven families are typically fresh-water in habitat—the Aegleidae, containing the single species Aeglea laevis, related to the Galatheidae, which inhabits streams in temperate S. America; the Atyidae, a family of Prawns from the tropical rivers and lakes of the New and Old World, and in the Mediterranean region. A number of Palaemonidae are found in fresh water, e.g. Palaemonetes varians in Europe and N. America, while several species of Palaemon occur in lakes, streams, and estuaries of the tropical Old and New World.

The expeditions of Moore and Cunnington to Lake Tanganyika brought back an exceedingly rich collection of Prawns, comprising twelve species, all of which are peculiar to the lake, and this is all the more surprising since Lakes Nyasa and Victoria Nyanza are only known to contain one species, Caridina nilotica, which ranges all over Africa and into Queensland and New Caledonia. The Tanganyika species, however, all belong to purely fresh-water genera, and do not afford any suggestion that they are part of a relict marine fauna. It would appear that they have been differentiated in the lake itself during a long period of isolation.

Two groups of Brachyura, viz. the Thelphusidae and the Sesarminae (a sub-family of the Grapsidae), are fresh-water. Thelphusa fluviatilis is an inhabitant of North Africa, and penetrates into the temperate regions of the Mediterranean, and is said to be exceedingly common in the Alban Lake near Rome. Both these families have representatives on land, e.g. Potamocarcinus in Central and South America, and certain species of Sesarma, and the closely related Gecarcinidae of the West Indies.

The remaining families to be dealt with are the two Crayfish families— the Astacidae and the Parastacidae—which live in rapidly moving rivers and streams, and occasionally in lakes. A few species of both families have taken to a subterranean mode of life, and excavate burrows in the earth, e.g. the Tasmanian Crayfish, Engaeus fossor. The distribution of the Crayfishes has long engaged the attention of naturalists. It was first seriously studied by Huxley, and has subsequently been followed up, especially in North America, by Faxon and Ortmann, but our knowledge of the South American and Australian forms is still very incomplete. The Astacidae inhabit the northern temperate hemisphere, the Parastacidae the southern temperate hemisphere, the tropical belt being practically destitute of Crayfishes. Of the Astacidae the genus Astacus (Potamobius), including the common Crayfishes of Europe, occurs in Europe and in North America west of the Rockies. The genus Cambaroides, which in certain respects approaches Cambarus, is found in Japan and Eastern Asia. The very large genus Cambarus, on the other hand, only occurs in North America east of the Rockies, so that Cambaroides occupies a very isolated position. The occurrence of a Cambarus, C. stygius, in the caves of Carniola, has been recorded by Joseph, so that it would appear that this genus had a much wider range formerly than now.

Of the southern temperate Parastacidae, Australia and Tasmania have the genera Astacopsis and Engaeus; New Zealand has Paranephrops, South America Parastacus, and Madagascar Astacoides. The last named genus is rather isolated in its characters, possessing a truncated rostrum and a highly modified branchial system, but it agrees with all the other Parastacine genera, and differs from the Astacidae in the absence of sexual appendages on the first abdominal segment, and in the absence of a distinct lamina on the podobranchiae. The largest crayfish in the world is Astacopsis franklinii, found in quite small streams on the north and west coast of Tasmania. Specimens have been caught weighing eight or nine pounds, and rivalling the European Lobster in size. Crayfishes appear to be entirely absent from Africa.

It seems reasonable to suppose that the two families of Crayfishes characteristic respectively of the northern and southern hemispheres have been independently derived from marine ancestors, which have subsequently become extinct. Their complete absence in the tropics is striking, and Huxley drew attention to the fact that it is exactly in those regions where the Crayfishes are absent that the other large fresh-water Malacostraca are particularly well developed, and vice versa. Thus the large fresh-water Prawns are typically circumtropical in distribution, while the South African rivers abound with River-crabs, which, in general, are found wherever Crayfishes do not occur.

A few of the more interesting features in connection with the distribution of fresh-water Crustacea have now been touched upon. With regard to the origin of this fauna, we can see that a number of the species are comparatively recent immigrants from the sea, working their way up the estuaries of rivers, a proceeding which can be observed to be taking place to-day in a district like the Broads of Norfolk. Others, again, but these are few, appear to be true relict marine animals left stranded in arms of the sea that have been cut off from the main ocean, and have been gradually converted into fresh-water lakes and seas. Such are, perhaps, Mysis relicta and the rich Gammarid fauna of Lake Baikal, a lake that, in the presence of Seals, Sponges, and other marine forms, has clearly retained some of the characters of the ocean from which it was derived. The majority of the fresh-water species, however, have probably been evolved in situ, and their origin from marine ancestors is lost in an obscure past. The Crustacean fauna of the Caspian Sea shows us in an interesting manner the effects of isolation and changes in salinity, etc., on the inhabitants of a basin which once formed part of the ocean. The waters of the Caspian Sea are not fresh, but they are on the average about one-third as salt as that of the open ocean. The Crustacea, described by Sars, belong to undoubtedly marine groups, e.g. the Mysidae, Cumacea, and Amphipoda Crevettina, but the remarkable feature of these Caspian Crustacea is the great variety of peculiar species representing marine genera which are very poorly represented in the sea, thus indicating that the variety of the fauna is not due to a great variety of species having been shut up in the Caspian Sea to begin with, but rather that, after the separation from the sea, the isolated species began to vary and branch out in the most luxuriant way—whether from lack of competition or owing to the changing conditions of salinity it is difficult to say. As an example, the Cumacea of the Caspian Sea are ten in number, all belonging to peculiar genera related to Pseudocuma, except one species which is included in that genus. These Caspian forms make up the Family Pseudocumidae, which contains in addition only two marine forms of the genus Pseudocuma (see p. 121). A very similar condition is found in the numerous Amphipods of the Caspian Sea. Considering the enormous changes that must have taken place in the distribution of land and water even during Tertiary times, it is astonishing that the fresh waters of the world do not contain more species in common with the ocean, but it must be considered that the limited area and comparatively uniform conditions of fresh-water lakes and streams would only permit a limited number of these forms to survive which could most easily adapt themselves to the changed conditions. And these would in all probability be the littoral species that were in the habit of passing up into the brackish waters of estuaries and lagoons, so that the uniform and limited nature of the fresh-water fauna can be accounted for to a certain extent by this hypothesis.

We have seen in dealing with the marine Crustacea of the littoral zone that the chief condition determining their distribution is temperature, and that the world may be divided into three chief areas of distribution for these animals, viz. the north temperate hemisphere, the tropics, and the south temperate hemisphere. It seems that the same division holds good for fresh-water Crustacea. We have already seen that the Crayfishes follow this rule, being practically absent from the tropics, and represented in the two temperate hemispheres by two distinct families, the Astacidae in the north and the Parastacidae in the south. Characteristic of the tropical belt are the absence of Crayfishes and the great development of Prawns and River-crabs. In the case of Entomostraca the great majority of the genera are cosmopolitan, especially those which live in small bodies of water liable to dry up, because these forms always have special means of dissemination in the shape of resting eggs which can be transported in a dry state by water-birds and other agencies to great distances; but those genera which inhabit large lakes are more confined in their distribution. The Copepod genus Diaptomus, characteristic of lake-plankton, ranges all over the northern hemisphere and into the tropics, but it is almost entirely replaced in the southern hemisphere by the related but distinct genus Boeckella, which occurs in temperate South America, New Zealand, and southern Australia, and was found by the author to be the chief inhabitant in the highland lakes and tarns of Tasmania, Diaptomus being entirely absent. Of the Cladocera there are a number of pelagic genera (e.g. Leptodora, Holopedium, Bythotrephes) entirely confined to the lakes of the northern hemisphere. The distribution of Bosmina is interesting. This genus is distributed all over the north temperate hemisphere in lakes and ponds of considerable size, not liable to desiccation; in the New World it passes right through the tropics into Patagonia, the chain of the Andes doubtless permitting its migration. In the tropics of the Old World it is unknown, but it turns up again, as the author recently found, as a common constituent in the plankton of the Tasmanian lakes. There is another instance of a group of Crustacea, characteristic of the north temperate hemisphere, being entirely absent from the tropics, at any rate of the Old World, but reappearing in the temperate regions of Australasia. The commonest fresh-water Amphipods in this region belong to the genus Neoniphargus, intermediate in its characters between the northern Niphargus and Gammarus, but grading almost completely into the latter. Both Niphargus and Gammarus are absolutely unknown from the tropics, but whether, like Bosmina, they occur in the Andes and temperate South America is not known; it seems, however, probable that they have reached Southern Australia by way of South America rather than through the tropics of Asia and Australia, where there is no range of mountains to permit the migration of a group of animals apparently dependent on a temperate climate. The other common fresh-water Amphipod in temperate Australia and New Zealand is Chiltonia, whose nearest ally is Hyalella from Lake Titicaca on the Andes, and temperate South America.

The Anaspidacea and Phreatoicidae, which are so characteristic of temperate Australia, and are generally of an Alpine habit, have never been found in South America, but the Anaspidacea are represented by numerous marine forms in the Permian and Carboniferous strata of the northern hemisphere, so that it is probable that this group reached the southern hemisphere from the north through America.

The distribution of the fresh-water Crustacea, therefore, in the temperate southern hemisphere affords strong evidence in favour of the view that the chief land-masses of this hemisphere, which are at present separated by such vast stretches of deep ocean, were at no very remote epoch connected in such a way as to permit of an intermixture of the temperate fauna of New Zealand, Australia, and South America. While this connexion existed, a certain number of forms characteristic of the northern hemisphere, which had worked through the tropics by means of the Andes, were enabled to reach temperate Australia and New Zealand. The existence of a coast-line connecting the various isolated parts of the southern hemisphere would, of course, also account for the community which exists between their littoral marine fauna. It is impossible to enter here into the nature of this land-connexion which is becoming more and more a necessary hypothesis for the student of geographical distribution, whatever group of animals he may choose, but it may be remarked that the connexion was probably by means of rays of land passing up from an Antarctic continent to join the southernmost projections of Tierra del Fuego, Tasmania, and New Zealand.

TRILOBITA

HENRY WOODS, M.A.

St. John’s College, Cambridge, University Lecturer in Palaeozoology