The Cambridge Natural History, Vol. 04 (of 10)

CHAPTER I. Crustacea—General Organisation

CRUSTACEA—GENERAL ORGANISATION

The Crustacea are almost exclusively aquatic animals, and they play a part in the waters of the world closely parallel to that which insects play on land. The majority are free-living, and gain their sustenance either as vegetable-feeders or by preying upon other animals, but a great number are scavengers, picking clean the carcasses and refuse that litter the ocean, just as maggots and other insects rid the land of its dead cumber. Similar to insects also is the great abundance of individuals which represent many of the species, especially in the colder seas, and the naturalist in the Arctic or Antarctic oceans has learnt to hang the carcasses of bears and seals over the side of the boat for a few days in order to have them picked absolutely clean by shoals of small Amphipods. It is said that these creatures, when crowded sufficiently, will even attack living fishes, and by sheer press of numbers impede their escape and devour them alive. Equally surprising are the shoals of minute Copepods which may discolour the ocean for many miles, an appearance well known to fishermen, who take profitable toll of the fishes that follow in their wake. Despite this massing together we look in vain for any elaborate social economy, or for the development of complex instincts among Crustacea, such as excite our admiration in many insects, and though many a crab or lobster is sufficiently uncanny in appearance to suggest unearthly wisdom, he keeps his intelligence rigidly to himself, encased in the impenetrable reserve of his armour and vindicated by the most powerful of pincers. It is chiefly in the variety of structure and in the multifarious phases of life-history that the interest of the Crustacea lies. Before entering into an examination of these matters, it will be well to take a general survey of Crustacean organisation, to consider the plan on which these animals are built, and the probable relation of this plan to others met with in the animal kingdom.

The Crustacea, to begin with, are a Class of the enormous Phylum Arthropoda, animals with metamerically segmented bodies and usually with externally jointed limbs. Their bodies are thus composed of a series of repeated segments, which are on the whole similar to one another, though particular segments may be differentiated in various respects for the performance of different functions. This segmentation is apparent externally, the surface of a Crustacean being divided typically into a number of hard chitinous rings, some of which may be fused rigidly together, as in the carapace of the crabs, or else articulated loosely.

Each segment bears typically a pair of jointed limbs, and though they vary greatly in accordance with the special functions for which they are employed, and may even be absent from certain segments, they may yet be reduced to a common plan and were, no doubt, originally present on all the segments.

Passing from the exterior to the interior of the body we find, generally speaking, that the chief system of organs which exhibits a similar repetition, or metameric segmentation, is the nervous system. This system is composed ideally of a nervous ganglion situated in each segment and giving off peripheral nerves, the several ganglia being connected together by a longitudinal cord. This ideal arrangement, though apparent during the embryonic development, becomes obscured to some extent in the adult owing to the concentration or fusion of ganglia in various parts of the body. The other internal organs do not show any clear signs of segmentation, either in the embryo or in the adult; the alimentary canal and its various diverticula lie in an unsegmented body-cavity, and are bathed in the blood which courses through a system of narrow canals and irregular spaces which surround all the organs of the body. A single pair, or at most two pairs of kidneys are present.

The type of segmentation exhibited by the Crustacea is thus of a limited character, concerning merely the external skin with its appendages, and the nervous system, and not touching any of the other internal organs. In this respect the Crustacea agree with all the other Arthropods, in the adults of which the segmentation is confined to the exterior and to the nervous system, and does not extend to the body-cavity and its contained organs; and for the same reason they differ essentially from all other metamerically segmented animals, e.g. Annelids, in which the segmentation not only affects the exterior and the nervous system, but especially applies to the body-cavity, the musculature, the renal, and often the generative organs. The Crustacea also resemble the other Arthropoda in the fact that the body-cavity contains blood, and is therefore a “haemocoel,” while in the Annelids and Vertebrates the segmented body-cavity is distinct from the vascular system, and constitutes a true “coelom.” To this important distinction, and to its especial application to the Crustacea, we will return, but first we may consider more narrowly the =segmentation= of the Crustacea and its main types of variation within the group. In order to determine the number of segments which compose any particular Crustacean we have clearly two criteria: first, the rings or somites of which the body is composed, and to each of which a pair of limbs must be originally ascribed; and, second, the nervous ganglia.

Around and behind the region of the mouth there is very little difficulty in determining the segments of the body, if we allow embryology to assist anatomy, but in front of the mouth the matter is not so easy.

In the Crustacea the moot point is whether we consider the paired eyes and first pair of antennae as true appendages belonging to two true segments, or whether they are structures sui generis, not homologous to the other limbs. With regard to the first antennae we are probably safe in assigning them to a true body-segment, since in some of the Entomostraca, e.g. Apus, the nerves which supply them spring, not from the brain as in more highly specialised forms, but from the commissures which pass round the oesophagus to connect the dorsally lying brain to the ventral nerve-cord. The paired eyes are always innervated from the brain, but the brain, or at least part of it, is very probably formed of paired trunk-ganglia which have fused into a common cerebral mass; and the fact that under certain circumstances the stalked eye of Decapods when excised with its peripheral ganglion can regenerate in the form of an antenna, is perhaps evidence that the lateral eyes are borne on what were once a pair of true appendages.

Now, with regard to the segmentation of the body, the Crustacea fall into three categories: the Entomostraca, in which the number of segments is indefinite; the Malacostraca, in which we may count nineteen segments, exclusive of the terminal piece or telson and omitting the lateral eyes; and the Leptostraca, including the single recent genus Nebalia, in which the segmentation of head and thorax agrees exactly with that of the Malacostraca, but in the abdomen there are two additional segments.

It has been usually held that the indefinite number of segments characteristic of the Entomostraca, and especially the indefinitely large number of segments characteristic of such Phyllopods as Apus, preserves the ancestral condition from which the definite number found in the Malacostraca has been derived; but recently it has been clearly pointed out by Professor Carpenter that the number of segments found in the Malacostraca and Leptostraca corresponds with extraordinary exactitude to the number determined as typical in all the other orders of Arthropoda. This remarkable correspondence (it can hardly be coincidence) seems to point to a common Arthropodan plan of segmentation, lying at the very root of the phyletic tree; and if this is so, we are forced to the conclusion that the Malacostraca have retained the primitive type of segmentation in far greater perfection than the Entomostraca, in some of which many segments have been added, e.g. Phyllopoda, while in others segments have been suppressed, e.g. Cladocera, Ostracoda. It may be objected to this view of the primitive condition of segmentation in the Crustacea that the Trilobites, which for various reasons are regarded as related to the ancestral Crustaceans, exhibit an indefinite and often very high number of segments; but, as Professor Carpenter has pointed out, the oldest and most primitive of Trilobites, such as Olenellus, possessed few segments which increase as we pass from Cambrian to Carboniferous genera.

The following table shows the segmentation of the body in the Malacostraca, as compared with that of Limulus (cf. p. 263), Insecta, the primitive Myriapod Scolopendrella, and Peripatus. It will be seen that the correspondence, though not exact, is very close, especially in the first four columns, the number of segments in Peripatus being very variable in the different species.

TABLE SHOWING THE SEGMENTATION OF VARIOUS ARTHROPODS ┌──┬──────────────┬───────────┬────────────┬─────────────┬────────────┐ │ │Malacostraca. │Limulus. │ Insecta. │ Myriapoda. │Peripatus.│ │ │ │ │ │ (Scolopen- │ │ │ │ │ │ │ drella). │ │ ├──┼──────────────┼───────────┼────────────┼─────────────┼────────────┤ │ 1│Eyes │Median eyes│Eyes │ │ │ │ 2│1st antennae │Rostrum │Antennae │Feelers │Feelers │ │ 3│2nd antennae │Chelicerae │Intercalary │ │ │ │ │ │ │segment │ │ │ │ 4│Mandibles │Pedipalpi │Mandibles │Mandibles │Mandibles │ │ 5│1st maxillae │1st walking│Maxillulae │Maxillulae │1st jaw-claw│ │ │ │legs │ │ │ │ │ 6│2nd maxillae │2nd „ │1st maxillae│1st maxillae │2nd jaw-claw│ │ │ │ „ │ │ │ │ │ 7│1st │3rd „ │2nd maxillae│2nd maxillae │1st leg │ │ │maxillipede │ „ │ │ │ │ │ 8│2nd │4th „ │1st leg │1st leg │2nd „ │ │ │maxillipede │ „ │ │ │ │ │ 9│3rd │Chilaria │2nd „ │2nd „ │3rd „ │ │ │maxillipede │ │ │ │ │ │10│1st ambulatory│Genital │3rd „ │3rd „ │4th „ │ │ │ │operculum │ │ │ │ │11│2nd „ │1st │1st │4th „ │5th „ │ │ │ │gill-book │abdominal │ │ │ │12│3rd „ │2nd „ │2nd „ │6th „ │6th „ │ │13│4th „ │3rd „ │3rd „ │6th „ │7th „ │ │14│5th „ │4th „ │4th „ │7th „ │8th „ │ │15│1st abdominal │5th „ │5th „ │8th „ │9th „ │ │16│2nd „ │No │6th „ │9th „ │10th „ │ │ │ │appendages │ │ │ │ │17│3rd „ │ „ │7th „ │10th „ │11th „ │ │18│4th „ │ „ │8th „ │11th „ │12th „ │ │19│5th „ │ „ │9th „ │12th „ │13th „ │ │20│6th „ │ „ │10th „ │Reduced limbs│14th „ │ │21│ │ „ │ │Cercopods │ │ │ │Telson │Telson │Telson │Telson │Telson │ └──┴──────────────┴───────────┴────────────┴─────────────┴────────────┘

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