CHAPTER VII.. Commensalism and Parasitism.

COMMENSALISM AND PARASITISM.

The term Symbiosis has been applied by naturalists to the phenomenon of the living together for mutual help or protection of different species of animals or plants. It is a well-known fact, to all those who have taken an interest in any large group of animals, that some species are nearly always associated with other species, belonging perhaps to a different class altogether, and very frequently mimicking them in form or colour. At first it might be thought that most of these cases could be dismissed as cases of parasitism; but when the careful observer notices that neither of the species is injured by the association, the conditions of the partnership are evidently very different to those of a blood-sucking parasite and its ungracious host.

Besides the words Symbiosis and Parasitism, the terms Commensalism and Mutualism have been applied to various cases of association of different species of animals; but with the increase of our knowledge of the habits of animals, it is becoming more and more difficult to classify all known cases under these four heads, and the words are consequently often used with widely different meanings.

It will be perhaps the best plan to adopt in this book, to avoid any attempt to give definitions of these terms until a few cases illustrative of each have been described.

One of the commonest objects of the sea-shore is the Hermit-crab. From the open mouth of what is apparently an empty shell a bundle of claws and legs may be seen to protrude; turn the shell over and it will scamper away into the deeper parts of a rock pool. This is an association of a living Crab with the shell of an animal that is dead; but if the Hermit-crab be extracted it will be seen that it has a soft and twisted tail, quite unlike that of the shore Crabs, and that it could not possibly live for any length of time without the shelter and protection afforded by the shell that it has appropriated to itself. The Hermit-crab in the course of its life increases in size, and when it gets too big for the shell it is living in, it goes in search of another a little bit larger and changes, until at last it attains to the size and dignity that requires a full-grown Whelk shell.

In the waters of our coast just beyond the low tide mark we often find that the shell containing a Hermit-crab bears a Sea-anemone which belongs to a species rarely found anywhere excepting in association with a Hermit-crab. Moreover, the Anemone is always seated in a definite position on the shell, so that its mouth is turned towards the jaws of the Hermit-crab when it is extended, enabling it to catch any morsels of food that escape the mouth of its comrade. When the Hermit-crab has grown too large for its shell, and moves into a new one, the Anemone moves too, and takes up the same position on the new shell that it occupied on the old one, and the companionship is continued in this manner throughout life.

The advantage of this arrangement to the Anemone is obvious, for it can not only obtain its food after the manner of the other Anemones, but it also gains a share of the food of the Hermit-crab. The advantage to the Crab is not so apparent, but it is probable that the Anemone, being very distasteful to many Fish and other animals, acts as a protector to it. The facts that Hermit-crabs are extremely shy, darting back into their shells when there is the slightest sign of danger, and that they are extremely good bait for many kinds of Fish, suggest very forcibly that they have many enemies among the inhabitants of the deep. Any such covering as that afforded by the Anemone, which hides to a great extent the character of the shell, would be of protective value, but when to that is added the fact that the Anemone, which affords this covering, is avoided as uneatable and distasteful by carnivorous Fish, there can be no doubt whatever of the assistance that it renders to the Crab in return for its board. If any of my readers are sceptical about the distastefulness of Sea-anemones I would ask them to think of any instance in which Sea-anemones are used for bait, and then to try the experiment of offering pieces of them to the Fish in an aquarium.

An observation by Prof. Möbius in the Indian Ocean affords another example of the use of Sea-anemones in this respect. He discovered a little Crab called Melia tesselata which carried about in each of its claws a Sea-anemone. When the Crab was alarmed it held them up in much the same way that a man holds a torch, as if it would call attention to the fact that it had these terrible weapons at hand. When the Anemones were removed it carefully searched for them, and held them up again when found, and even when the Anemone was cut into pieces the Crab diligently collected them, arranged them as far as possible in their proper places, and held them up together.

But Sea-anemones are not the only animals that seem to be generally distasteful to Fish. Many of the Sponges are free from attack, and could serve as a protection to the Hermit-crabs. On our own coast a small brown Sponge is not infrequently brought up in the dredge surrounding and protecting a Hermit-crab; and hidden somewhere in the substance of the sponge, there may always be found a small shell which lies at the end of the hole in which the Crab lives.

This association is, from the Crab’s point of view, a more advantageous one than that with the Sea-anemone, for it does away with the necessity of any changes of shell, the Crab and the Sponge growing up together. The history of the companionship is probably as follows:--A small Hermit-crab takes for its shelter a small Gastropod shell, and upon this shell a Sponge larva settles, grows and spreads, until it surrounds the whole of it except the hole from which the Crab emerges. As the Sponge grows still further in thickness the margin over-lapping the aperture of the shell expands, leaving a conical cavity leading from the exterior to the shell, surrounded, of course, by Sponge structure, in which the Crab lives. Thus as the Hermit-crab increases in size it is ever provided with a wider hole to accommodate its body by the growth of the Sponge, and the little shell wholly deserted remains as a token of the past history of the pair. But in the later stages of growth a third creature is taken into the partnership, in the person of a small segmented Worm which lives in the hole with the Hermit-Crab. The need for this third person seems to be one of a sanitary character. The cleanliness of the Hermit-crab, which has no sponge to protect it, is provided for by the simple expedient of frequent changes into a new home. In this case it is arranged for by taking into the home, on what we may call board wages, an efficient scavenger.

In this remarkable association, then, no less than four species belonging to four different groups of animals are concerned. First of all there is the Gastropod Mollusc, which forms a shell for the Crustacean Hermit-crab to commence life in, then there is the Sponge which protects, and afterwards forms a shelter and home for the Hermit-crab, and lastly, there is the Annelid worm, which helps in its way to keep the house clean in return for the scraps of food that fall from the head partner’s table.

A very similar association has recently been described by Bouvier from the Gulf of Aden and Red Sea waters. A number of simple solitary Corals were thrown into an aquarium by a French naturalist, some falling on their sides and some on their crowns, but he noticed that, after the lapse of some time, they were all in the erect position again with their crowns of tentacles expanded in the water. On carefully watching them he observed that at the base of each Coral there was a little hole from which emerged a small unsegmented Worm, belonging to a family that usually exhibits sand-burrowing propensities. These Worms were found to be the agents which restored the Corals to their erect positions. The advantage of this arrangement to the Worm was two-fold: it brought it into direct contact with the sand in which it searches for its food, and, at the same time, it brought the Coral into such a position as to hide and protect it from its enemies above in a most effectual manner. To the Coral it was obviously an advantage, in that it placed it in a position to expand its tentacles in search of the food it seeks in the water and prevented a death from suffocation. A more minute investigation of the Coral, however, revealed the facts that hidden in its substance there was a small Gastropod shell on which we may suppose both the Coral larva and the Worm settled when the partnership began, and that in association with the Worm there was a small bivalve Mollusc which probably acts as a scavenger in the manner of the Worm in the last mentioned case. Here again, then, there are three different species living together to their mutual advantage and commencing their association on the shell of a fourth species belonging to a different class of animals. What words can we apply to these associations? The Hermit-crab and the Anemone feed at “the same table” and therefore they afford a case of “Commensalism”; the Coral and the Worm are of advantage to one another, the former in shielding and protecting the latter and the latter in keeping the former in an upright position, but as they do not feed “at the same table” it is not a case of “Commensalism” but rather one of “Mutualism.”

There are many cases, however, of the association of animals in which, although the advantage to one of the partners is clear, it is extremely difficult to say what benefit is derived by the other.

Living in a tube on our coasts is a very common Worm called Sabella, and at the mouth of the tube a little Polyp may frequently be found which has received the fanciful name of the “Household god of the Sabellids” (Lar Sabellarum). The Polyp undoubtedly benefits by the currents of water which the Worm sets up when feeding, but it is difficult to see what advantage, if any, the worm gains from the presence of the Polyp.

Again, some of the Trepangs are frequently inhabited by a little Fish called Fierasfer, which comes out from time to time to feed and “take the air,” but rapidly retreats into the body of the Trepang on the slightest alarm.

The large stinging Sea-anemones of the Coral often afford protection of a similar kind to a little Fish. Saville Kent gives a beautiful picture of a little bright red Fish swimming about on the disc of a large purple Sea-anemone, and he says that it darts into the mouth when alarmed. On our own coasts we may often observe a number of little Fish generally belonging to the Cod-family swimming round the disc and tentacles of the large Jelly-fishes, and these, when frightened, swim vigorously toward the under surface of the umbrella and seek security there. Sometimes as many as a hundred or more of them may be seen hovering round one large Jelly-fish, and we can hardly estimate how valuable to our sea fisheries is the protection afforded by these great Medusæ to the young Fish-fry. (See Frontispiece.)

It was not my purpose in writing this book to point out the practical value of scientific investigation, but this history of the Jelly-fish and Codling cannot be passed without comment. The Jelly-fish might readily be regarded by the ignorant not only as useless to man, but, in so far as they sometimes choke his fishing nets and sting his hands and arms, a positive nuisance to him. Scientific investigation when pursued by properly qualified persons for its own sake, and not for any definite commercial results that may possibly come out of it, frequently reveals facts of the utmost importance, such as the one that has just been mentioned.

There are some other cases of association which would on first consideration be called undoubtedly cases of parasitism, but as this term has been used somewhat vaguely in popular English, it would be well, before proceeding further, to place before the reader a definite statement of the sense in which the word is used in this book.

In many of the cases that we have mentioned hitherto of animals living together, no apparent injury is inflicted upon either of the associates, but a very definite and decided advantage accrues to each of them, by the association.

In other cases, however, whilst no apparent injury is inflicted on either, the advantage of the partnership falls entirely to one of them.

In a third set of cases one of the associates feeds upon the blood or tissues of the other without rendering it any service in return, and consequently inflicts either temporary or permanent injury. These are cases of parasitism. In such an association the animal that inflicts the injury is called the “parasite,” and the one that receives it, the “host.”

One difficulty the naturalist has to contend with in trying to use these terms correctly is that of finding out whether in any particular case an injury is inflicted or not; another is that of determining whether those animals should be called parasites which injure, alter, or destroy the tissues of their hosts without feeding upon them.

A few cases will throw more light upon the subject than any further discussion of the difficulties surrounding the application of these terms.

One of the commonest Corals to be found upon the coral-reefs of both the Old and New world is one called Millepora. In the Millepores of the Pacific region we very frequently find a number of Barnacles (called Pyrgoma milleporæ) so deeply buried in the substance of the Coral that their presence is indicated only by a small oval hole on the surface. There can be little doubt that in the course of the growth of these Barnacles they distort, if they do not actually destroy, some of the connecting canals of the Coral in their immediate neighbourhood, but their food is derived entirely from the water that surrounds the Coral and not any portion of it from the cells or tissues of the Coral-polyps themselves.

There is a great deal of difference in the Millepores from one and the same coral-reef, in the extent to which the Barnacles have attacked them. In some specimens large areas of the Coral are beset with the little holes, in others only one or two may be found on the whole colony, whilst others again are quite free from them. Now when we compare carefully the anatomy of those Millepores with the Barnacles and those without them, no single sign or symptom can be found that the vigour or strength of the former is in any way impaired. If then there is no evidence that the Barnacles are parasitic, in the sense that they are injurious to the Millepores, we must next inquire whether they could possibly be of any service to them.

The polyps of the Millepores feed after the manner of the polyps of other Corals, upon minute organisms floating in the sea; these they paralyse and capture by means of tentacles bearing stinging cells. The food is in the ordinary course brought within reach of the tentacles by the tides that sweep over the reefs. The Barnacles also feed upon minute organisms of the same kind, but they are provided with six pairs of long feathery legs which by a curious vibratory movement create currents in the water. When there are many Barnacles in close proximity to one another it is quite probable that the water is considerably disturbed by these currents, and the constant and rapid flow of fresh water bearing food-organisms benefits, not only the Barnacles, but also the Millepore polyps in their neighbourhood.

Thus the Barnacles may be a benefit to the Millepores in which they live. It cannot be asserted, however, that this probability is a proved fact. A great deal more knowledge about the rate of growth of the Corals which are and are not affected, must be acquired before such an assertion could be made: but the probability that the Barnacles may be of service is sufficient to cause us to hesitate before branding them with the epithet of “parasites.”

This particular case, which has been given above in some detail, may be regarded, in a sense, as a test case, because other animals besides the Barnacles, which gain their food by producing currents, are found in Corals. Such are the tubicolous Worms, bivalve Molluscs, and certain Sponges. So plentiful are these on the older branches of some Corals, that quite a rich Fauna belonging to several groups of animals may be found by carefully studying them. These might all be dismissed as parasites by the non-inquisitive mind, but many of them, at any rate, may be regarded by the more cautious naturalist as not injurious, and others perhaps as positively beneficial to the Coral on which they live. There is a very curious case of symbiosis mentioned by Semper, which may be related here as similar in some respects to those above quoted.

On the shores of the Philippine Islands and in other parts of the Pacific Ocean there is a very common coral named Seriatopora. It is composed of numerous delicate branches, terminating in fine pointed extremities, forming hemispherical shrub-like masses, six or eight inches in diameter. Semper noticed that on some of the branches of these Corals there were little heart-shaped swellings, which had the appearance of malformations or structures corresponding to the galls on the leaves and branches of trees. Each of these swellings contained a cavity, communicating with the exterior by two minute holes, in which there was imprisoned a small Crab.

By the examination of a large number of specimens, Semper came to the conclusion that the history of these structures was somewhat as follows. The young Crab, when it settled down on the branch, produced an irritation which in some way caused a gall-like growth of the tissues of the Coral. This growth continued until it formed at first a case or sheath for the protection of the Crab, and eventually, as the Crab increased in size, a cage from which it could not do more than protrude its tentacles and claws when feeding.

It seems very improbable that these cage-like swellings upon the branch can be of any great disadvantage to the Coral. It is true that they destroy the beautiful symmetry of the branch, and give it a distorted and diseased appearance; but this is only an æsthetic disadvantage, which does not probably count for much in the struggle for existence on the Coral-reef. To the Crab the arrangement is undoubtedly an advantage, as it gives it a secure position, free from the attack of its ordinary foes, where food is probably abundant and easily obtained.

The skin of Whales is often beset with Barnacles; in fact some species of them are found nowhere else but on these Mammals. They are usually deeply embedded in the skin, only a small round hole through which the legs can be protruded, communicating with the exterior. These Barnacles do not feed upon the tissues and juices of the Whale, but, in the usual manner of the non-parasitic Barnacles, upon organisms that swim freely in the water. The advantage to the Barnacles is obvious, as the movements of the Whale through the water must bring them in reach of constant fresh supplies of food, but the benefit to the Whale is not so clear. It cannot be supposed for a moment that the Barnacles assist the Whales in their search for food, nor can they be regarded, when present in great numbers, as a protection to the skin by the strength afforded by their thick calcareous shells; at the same time there is no reason to suppose that their presence is an inconvenience or in any way harmful to the Whales.

These cases of animals bearing on their bodies other creatures which are not in the strictest sense of the word parasites, are but instances of a phenomenon that is very widely spread among marine organisms. There are many cases, however, in which plants and inorganic foreign bodies play a very important part in the economy of animals.

In our chapter on the free-swimming organisms of the ocean, mention has been made of the delicate and beautiful creatures called Radiolarians. Many years ago it was discovered that each of these animals bears in its protoplasm a number of little cells, which from their colour received the name of “the yellow cells.” It was clear from observation and experiment that they were neither organs nor products of the Radiolarian, but independent organisms belonging to the Vegetable Kingdom.

More recently cells similar to these have been found in many of the Corals, in Worms, and other animals, and there can be no doubt now that when present they perform very important physiological functions which materially assist their host in its growth and development.

So numerous are these “yellow cells” in some Polyps and so important must be their influence on their vital processes, that it may be confidently asserted that the Polyps could not continue to exist for long without them. In the genus Millepora, for example, no single specimen and no single fragment of a specimen that I have examined was devoid of them; and although the numbers vary considerably the most superficial canals of this Coral may in all cases be described as crowded with “yellow cells.” But as the “yellow cells” are certainly of great physiological importance to the Millepore, it is equally certain that the secretions and the protection afforded by the Millepore are of extreme importance to the “yellow cells.” In fact it is not going too far to say that the Millepore and its “yellow cells” are dependent upon one another for their existence, and the naturalist might say with a great deal of truth that this particular Coral is not, strictly speaking, animal in nature, but rather an animal and vegetable combination.

Many years ago there was a bitter controversy among learned men on the question of the animal or vegetable nature of Corals. The great naturalist Linnæus, who was appealed to for his support by both parties to the controversy, took up a middle position, asserting that they were partly of the nature of animals and partly of the nature of plants, and hence the term “zoophytes,” i.e. animal-plants, came to be applied to them. There can be no doubt that in the end the position in the controversy, assumed by Linnæus, became untenable, and the supporters of the animal view of zoophytes won all along the line. It is curious, therefore, that we are now in a position, not to support the view of Linnæus, but to assert that some Corals are essentially a combination of animals and plants.

Plants are of use to marine animals, however, in another manner. Mention has already been made of the way in which many animals resembling in colour, and even in form, certain kinds of sea-weeds, escape the attention of their enemies and hide for safety among the plants they simulate. Sometimes, however, the weeds will grow upon the shells of the animals, and thus hide them even more effectually. One of the most remarkable instances occurs in a Spider-crab that is common upon our own coasts. The Inachus, as it is called, is usually covered with a little forest of algæ, which do not grow there naturally, but are actually placed on the carapace by the Crab itself. If the plants be scraped off artificially the Crab will go in search of fresh ones, carefully chew the bases until they are soft, and then deliberately decorate the carapace with them as before.

There are some Molluscs that artificially decorate themselves with little shells and other objects in such a manner as to completely hide their general form. One of the most remarkable instances of this occurs in the Gastropod Xenophora, which covers its own shell with numbers of others belonging to a smaller species, so that in the natural state it has the appearance of a conglomerate of shells. The manner in which the smaller shells are fixed has not yet been described, but from the orderly arrangement which they exhibit in some cases there can be little doubt that they are deliberately placed in position by the Gastropod itself and not attached by accidental contact.

In both these cases it is clear that the reason for the phenomena described is that of affording a covering or mantle, which hides or obscures the real form and character of the living animals.

Many of the Worms use little bits of shell and grains of sand to build up a tube for the protection of their bodies. One of these--the Terebella--is very common on our shores, the sandy tubes ending in a tuft of fine filaments, and decorated all over with tiny little stones or shells, projecting an inch or two from the surface of the sand. In some localities these tubes may be found in thousands when the tide is low.

Another form--Pectinaria--constructs much firmer tubes, which retain their cylindrical shape after the death of the animal. In the process of construction this Worm must carefully select the grains of sand, for when the tube is examined with a magnifying glass the particles will be seen to be of almost exactly the same size, and arranged in their places with a mathematical precision.

But Worms are not by any means the only animals that use the sand in this manner for the protection of their bodies. There are some kinds of Polyps, belonging to the family Zonathidæ, a peculiar group of Sea-anemones, in which the body-wall is considerably strengthened by foreign bodies of various kinds. The Zoanthus does not, like the Terebella-worm, form a tube or case in which the body can freely move up or down, but sticks the grains of sand into its skin, so that they become in the older forms deeply buried in the tissues and give a considerable support to the body-wall.

The Cerianthus--another Sea-anemone--forms a tube which is partly composed of a matted network of stinging threads, and partly of the mud in which the animal lives.

The use of foreign inorganic substances for the protection or concealment of animals is not, strictly speaking, however, a part of the subject-matter of this chapter, which was intended for the consideration of the associations of two different kinds of living organisms.

The subject of Parasitism must now be considered, a subject which presents so many features of interest that it is possible here only to touch on a few points of general importance. It is a well-known truism to say that Parasitism, whether in human society or in animal life, leads to degeneration; but there are degrees of parasitism among animals, and consequently degrees of degeneration exhibited by animal parasites. We may roughly divide them into two classes, the outside or skin parasites and the internal parasites, the latter being invariably far more modified in structure and in development than the former.

Among the terrestrial animals we find a great number of external parasites, such as the Fleas and the Bugs, which are only slightly modified, as in the loss of their wings, owing to their habits, and can live an active, if not a very prosperous, life for some length of time apart from the society of their hosts. There are others, such as the Mosquitoes, Ticks, and Leeches, which are only occasional parasites; that is to say, they will suck the blood of another animal when the opportunity is presented, but failing that, are able to continue their life and their race independently. It is not surprising, however, that the terrestrial Vertebrates should be thus subjected to the attacks of these parasites, as their feathery or hairy skin affords a shelter and a foothold, from which the efforts of their hosts to dislodge them are exercised in vain.

The skin of Fish, although covered with over-lapping scales, is smooth and slippery, and with the rapid movement through the water many of the forms of parasites of the types we meet on land would, if they existed in the sea at all, find a difficulty in securing an attachment. It is, however, provided with another means of defence against skin parasites, in the possession of numerous mucous glands which keep the body bathed with a slimy fluid.

Everyone must have noticed the slime that exudes from freshly killed Fish, and if the finger be pressed along the skin it is possible to see the openings of the glands as the slime is squeezed out. In the majority of Fish the openings of the glands are most easily seen on the jaws and the flap of the gill cover.

We must remember that the sea is in most places teeming with the larvæ of Worms, Barnacles and Zoophytes, and the spores of Algæ and Fungi of various kinds. Logs of wood, the iron supports of piers, and the bottoms of ships become covered with various fixed forms of animal and vegetable life when submerged in the sea-water for even a few weeks. How is it, then, that the bodies of the Fish are usually so clean and wholesome? The answer to this question is probably to be found in the slime which, passing continuously over the skin, removes the larvæ and the spores before they can secure a firm attachment.

The Crabs, Lobsters and other Crustaceans free themselves from their skin parasites at every moult, but in some of the large, old Lobsters and Crabs that are caught a considerable number of Worms, Barnacles and weeds are frequently found firmly-fixed to the carapace and claws. The Limpets and Winkles of our rock pools are often covered with a little forest of Algæ.

The shells of other Molluscs are, however, kept remarkably clean, and the method by which they destroy the spores, etc. that settle upon them is not yet fully understood.

One of the most serious of the external parasites is the Hag-fish. This remarkable animal is eel-like in shape, although very different indeed, anatomically, from all the true Fishes, and buries its head in the skin of the Cod and other Fishes as it feeds upon their flesh. In some cases the whole body of the Hag gets inside the host, and it thus becomes an internal parasite. It causes an immense destruction of valuable food fish in some districts.

Closely related to the Hag is the marine Lamprey, which fastens itself to Salmon by its suctorial mouth, causing considerable wounds. This parasite sometimes reaches to a length of two feet, and is often carried many miles up the river by the host to which it is attached.

Most of the Leeches occur either in fresh water or in damp forests and marshy places. There is one, however, named Pontobdella, which is found only in sea-water. It is difficult to give an exact statement as to its size, because, like all its relations, it is capable of very extensive movements of expansion and contraction, but the Pontobdella is large for a Leech, and when moderately contracted it may be two or three inches in length. The body of this Leech is covered with small tubercles, and it has a large round sucker at each end. Its favourite hosts are the Sharks and Rays, but as it usually drops back into the water when these Fish are hoisted on to the deck it is not very commonly seen in the fishermen’s boats.

The most common external parasites of Fish are the Fish-lice. Most of these are little Crustaceans, belonging to a group which includes the Wood-louse. They have curiously flattened bodies, provided with short, bent legs, terminating in sharp hooks, by which they adhere to the body of the Fish and crawl about over the skin. Some of these parasites seem to prefer the tongue as a resting-place, the genus Glossobius, for example, being found in this position on the Flying-fish of both the Pacific and Atlantic Oceans. In Glossobius we find a very remarkable difference in size and form between the males and females, a condition of affairs which is of very common occurrence among the parasitic Crustaceans. The male in this particular case is so small that it is entirely concealed beneath the tail of the female. In another genus a still more interesting condition has been observed, the small young forms which are males growing up, and changing in later life into females.

There is a remarkable parasite called Sacculina which may sometimes be found on the under side of the tail in Crabs. In shape it is like a small pea or bean, and is attached to its host by a number of root-like processes, which penetrate through the skin and burrow deeply into the subjacent tissues. It would be quite impossible to tell to what group of animals this parasite belongs by the study of the adult form alone. It is, in fact, little more than a skin full of eggs. When the development of the eggs is watched, however, it is observed that the young Sacculina as it is hatched is very much like the Nauplius larva of a Barnacle. The later stages of the development prove that whatever may happen to the adult the Sacculina must be related to the group of the Cirripedia. Later on it is found that the females settle down on a Crab, lose all their limbs and other Cirripedian characters, and finally degenerate into a mere palpitating sac of eggs.

The males never pass beyond the second stage of development known as the Cypris stage. Several of them may usually be found attached to the female, and although they always remain extremely minute they do not lose entirely their Crustacean features.

For those who are in search of parasites, however, there is no more fruitful ground than the gills. That these organs should be a good place for attack is not surprising, when we consider that to maintain the respiration of the animal a constant flow of sea-water over them must be kept up, and this must bring with it many larval forms which may take the opportunity to attach themselves as they pass through the meshes of the gill filaments. Moreover, it is in the gills particularly that the blood current comes into closest contact with the water, and it requires but a little puncture on the part of the young parasite to reach a constant supply of this nourishing fluid.

It is in the gill-chambers that we find most frequently representatives of that interesting group of animals, the parasitic Copepods.

It would be difficult to recognise them as Copepods if we were to judge by their adult characters alone. Unlike the brisk, brightly-coloured creatures with long rowing antennæ that we have described above as living a free life in the surface waters of the ocean, these parasites have a white sac-like body, with short blunt processes representing the legs, no eye, and generally two long thread-like bags of eggs attached to the sides of the rudimentary tail. As we found in the case of the Sacculina, the true zoological position of these parasites can only be determined by reference to their developmental history.

In the gill-chamber of the Prawns we find a very much modified parasite, which is closely allied to those skin parasites of Fishes mentioned above. Many of my readers may have noticed that in some Prawns there is a wart-like swelling on one side of the neck. If the skin be removed it will be observed that this is in reality a cup-shaped protrusion on the wall of the gill-chamber covering a little, flat, soft animal. In past times it was thought that this was a young flat fish, and a wonderful story of its development was fabricated on the strength of this error. It is now known to be one of these extremely degenerate Isopod parasites called Bopyrus.

It is a curious fact that there is very rarely indeed more than one of these parasites on a single Prawn. If there is one in the right gill-chamber there are none in the left, and vice versâ. It is difficult to find a satisfactory explanation for this, for it is not at all probable that, during the lives of the many hundreds of Prawns that have been examined, only one larva has passed through the gill-chamber of each individual. The explanation must be looked for in some hitherto unknown influence which the parasite has upon the constitution of the host, rendering it unsuitable for the attachment of another Bopyrus of the same habits. The case is by no means unique. There are several instances of Fish and other animals that bear one, and never more than one, parasite of a particular species.

A few words must now be added about the internal parasites of marine animals. The subject is really an immensely wide one; for the intestines, body cavities, and even blood-vessels of Fish are liable to the attacks of many different forms of Flukes, Tape-worms, and other kinds of parasites which are not even known by name, perhaps fortunately, to the general public.

The life-history of some Flukes that occur in terrestrial animals has been satisfactorily worked out, and we know that, in most cases, they must infest two different hosts before they can reach maturity. The first of these hosts is usually an Invertebrate, and the second a Vertebrate animal. Moreover, it is known that the larvæ are extremely particular in their choice of the first host, attacking one species, and one species only, of Snail or Slug, or whatever Invertebrate its first host may be. If the first host dies a natural death or is swallowed by any other animal than the parasite’s proper second host, it--that is to say, the parasite--dies. It seems probable that the Flukes that infest the intestines of marine animals pass through some similar life-history, but owing to the great difficulties that confront the observer their development has not yet been thoroughly investigated. Similarly the life-histories of the Tape-worms, with which a very large number of marine animals are infested, are not yet known to us. It is comforting to know, after looking through the volumes of papers on these internal parasites of marine animals, that none of them have been shown to be, even occasionally, parasitic upon man, and we can continue our Fish diet without any misgivings on that score. An exception must, however, be made to this statement for the semi-marine Salmon and Sturgeon, which are suspected of being the first hosts of a human Tape-worm.

It is perhaps unsatisfactory to dismiss the internal parasites of marine animals with so few words, but I feel compelled to do so, not only because I have nearly outrun the limits of space, but because we possess so little positive information on the subject, which is of greatest interest to us here, of their developmental history. Lists of species infesting different Fish and Whales could be published, a statement of the points of anatomical importance which distinguish the families could be written, but they would present few features of interest to the general reader.

It may be well to point out before the chapter is closed, however, that there is probably no branch of our subject that is so little known and presents such a wide and important field for future investigation as the life-histories of these marine parasites.