The Life of Crustacea Part 1

The Life of Crustacea.

by William Thomas Calman.

PREFACE

This sketch of the Natural History of the Crustacea deals chiefly with their habits and modes of life, and attempts to provide, for readers unfamiliar with the technicalities of Zoology, an account of some of the more important scientific problems suggested by a study of the living animals in relation to their environment.

I am indebted to the Trustees of the British Museum for leave to reproduce certain figures prepared for the "Guide to the Crustacea, Arachnida, Onychophora, and Myriopoda exhibited in the Department of Zoology"; also to Sir Ray Lankester, K.C.B., F.R.S., and to Messrs. A.

and C. Black for the use of a number of figures from my volume on Crustacea in the "Treatise on Zoology," edited by Sir Ray Lankester.

The source of these figures is indicated in the explanation attached to each. Of the remaining ill.u.s.trations, some are reproduced from photographs of specimens in the collection of the British Museum; the others have been drawn from Nature, or copied from the original figures of various authors, by Miss Gertrude M. Woodward, to whom I am much indebted for the care and skill which she has given to their preparation.

W. T. C.

CHAPTER I

INTRODUCTORY

Everyone has some acquaintance with the animals that are grouped by naturalists under the name Crustacea. The edible Crabs, Lobsters, Prawns, and Shrimps, are at least superficially familiar, either as brought to the table or as displayed in the fishmonger's, and the most un.o.bservant of seaside visitors must have had his attention attracted by living specimens of some of the more obtrusive species, such as the common Sh.o.r.e Crab. Many, however, will be surprised to learn that the Barnacles coating the rocks on the seash.o.r.e, the Sand-hoppers of the beach, and the Woodlice of our gardens, are members of the same cla.s.s.

Still less is it suspected, by those who have not given special attention to the subject, that the living species of the group number many thousands, presenting strange diversities of structure and habits, and playing important parts in the general economy of Nature.

In addition to those just mentioned, a few Crustacea are sufficiently well known to be distinguished by popular names, such, for example, as Crayfish and Hermit Crabs, but for the vast majority no names are available except those of technical zoology. In the following pages, therefore, while technical terms have been introduced as sparingly as possible, the unfamiliarity of the animals themselves makes it needful to use many unfamiliar names.

In the cla.s.sification of the Animal Kingdom, the Crustacea form one of the divisions of a comprehensive group, or Phylum, known as Arthropoda.

The typical members of this group have a more or less firm external skeleton, the body is divided into segments, there are jointed limbs, and some of these are modified to serve as jaws. The chief divisions or cla.s.ses of the Arthropoda are--(i.) _Insecta_, including b.u.t.terflies, Moths, Bees, Wasps, Flies, Beetles, and the like; (ii.) _Chilopoda_, or Centipedes; (iii.) _Diplopoda_, or Millipedes[1]; (iv.) _Onychophora_, including the curious worm-like _Peripatus_; (v.) _Arachnida_, or Scorpions, Spiders, Mites, and their allies; and (vi.) _Crustacea_.

[1] The Chilopoda and Diplopoda are sometimes regarded as forming a single cla.s.s--Myriopoda.

It is not easy to summarize in a few words the characters common to all Crustacea, and distinguis.h.i.+ng them from the other groups of Arthropoda.

As a rough guide to cla.s.sification, it is useful to remember that an Insect can generally be recognized by having three pairs of walking legs, an Arachnid by having four pairs, and a Centipede or Millipede by having a great many pairs, all nearly alike. The Crustacea, on the other hand, show great diversity in the number and arrangement of their walking or swimming legs, but they rarely show any special resemblance to those of the other large groups of Arthropoda. Thus, for example, a common species of Woodlouse, _Armadillidium vulgare_, is very similar at first sight to the Millipede _Glomeris marginata_, but it has only seven pairs of walking legs, while the Millipede has seventeen or nineteen pairs.

More precisely, it may be said of the great majority of Crustacea that they are aquatic animals, breathing by gills or by the general surface of the body, having two pairs of "feelers," or antennae, on the front part of the head, and at least three pairs of jaws. Exceptions to each of these statements will be mentioned in later chapters in dealing with parasites and other highly modified types. In such cases, however, the larval or young stages afford indications of affinity, and comparison with less modified forms enables us to trace a connection with the typical Crustacea.

The best way to form a conception of a group of animals, however, is not to attempt in the first place to define its limits, but to begin by studying the structure of some typical and central species, and afterwards to note the divergences from this type presented by other members of the group. Speaking very generally, it may be said that these divergences are of two kinds. On the one hand there are characters that have no apparent relation to the animal's habits and mode of life, and on the other hand there are modifications of structure which are more or less plainly of use to the animal. It is to characters of the former cla.s.s that we look for evidence of an animal's affinities, and it is upon them that our systems of cla.s.sification are chiefly based. The characters of the second cla.s.s--"adaptive" characters, as they are called--become of importance when we study the animal "as a going concern," so to speak, and endeavour to understand how its life is carried on in relation to its surroundings.

In pursuance of this plan of study, the next chapter will be devoted to a description of the Common Lobster as a type of the Crustacea. In the third chapter a survey of the cla.s.sification of the group will be given; since, however, the characters on which the cla.s.sification is based cannot be explained fully without entering into technical details which are beyond the scope of this work, this survey will be restricted to what is necessary for comprehension of the succeeding chapters. In the fourth chapter some account is given of the young or larval stages of Crustacea, and of the changes they undergo in the course of development.

In the next five chapters the Crustacea are cla.s.sified according to their habitats, and those living in the shallow waters, the depths, and the surface of the ocean, in the fresh waters, and on land, are discussed in turn; while a separate chapter is devoted to the curious forms that live as parasites on, or as a.s.sociates with, other animals.

The last two chapters deal respectively with the Crustacea as they affect man, and with the past history of the group as revealed by fossil remains.

CHAPTER II

THE LOBSTER AS A TYPE OF CRUSTACEA

The most noticeable feature distinguis.h.i.+ng the Lobster[2] (Fig. 1) at first sight from other familiar animals is the jointed sh.e.l.ly armour that encases its body and limbs. Over the fore part of the body this armour is continuous, forming a s.h.i.+eld, or _carapace_, which projects in front, between the eyes, as a toothed beak, or _rostrum_; on the hinder part--the tail, or _abdomen_--it is divided into six segments, or _somites_, connected with each other by movable joints. Each of these somites carries on the under-side a pair of fin-like limbs, or _swimmerets_, the last pair of which (uropods) are much larger than the others, and are spread out at the sides of a middle tail-plate, or _telson_, forming what is known as the _tail-fan_. Since the fore part of the body also has a series of paired limbs, constructed, as will be shown later, on the same plan as the swimmerets, it is concluded that this part also is built up of somites, which have become soldered together. That this conclusion is correct is shown by comparison with some of the lower Crustacea in which this part of the body is divided up into eight separate somites, like those of the abdomen, each carrying, in place of the swimmerets, a pair of walking legs. In front of these eight somites, forming what is called the _thorax_, is the head--a part of the body which is never, in any Crustacean, broken up into distinct somites, but which, since it carries five pairs of appendages, must consist of at least five somites. The part of the body covered by the Lobster's carapace includes both the head and the thorax, and is known, therefore, as the _cephalothorax_. It is necessary to bear in mind that the parts of the body to which the names head, thorax, and abdomen, are applied in Crustacea are by no means exactly equivalent to those which bear the same names in Insects, for example, and that, beyond a rough similarity in position, they have no sort of relation to the parts so named in the body of a vertebrate animal.

[2] The account given here of the structure of the Lobster applies almost equally well to the River Crayfish or the Norway Lobster. The student is recommended to follow the description with a specimen of one of these animals before him.

[Ill.u.s.tration: FIG. 1--THE COMMON LOBSTER (_Homarus gammarus_,) FEMALE, FROM THE SIDE. (From British Museum Guide.)]

There are altogether twenty pairs of appendages attached to the body of the Lobster. In front of the head are the stalked _eyes_ (of which the nature will be discussed later) and two pairs of feelers--the _antennules_ and _antennae_ (sometimes called the first and second antennae). Near the mouth on the under-side of the head are three pairs of jaw-appendages--the strong _mandibles_ and the flattened, leaf-like _maxillulae_ and _maxillae_. Following these are the appendages of the thorax, of which the first three are intermediate in form between the true jaws and the legs, and are therefore termed foot-jaws, or _maxillipeds_. The remaining five pairs of thoracic limbs are the _legs_, the first pair forming the large and powerful pincer-claws, or _chelipeds_, while the others are the walking legs. The six pairs of swimmerets on the abdomen have already been mentioned.

If one of the somites of the abdomen be separated from the others, it will be seen (Fig. 2) to consist of a sh.e.l.ly ring, to which the two swimmerets are attached, wide apart, on the under-side. The arched upper part of the ring is known as the _tergum_, and the more flattened under-part as the _sternum_. On each side the tergum overlaps the sternum, and hangs down as a side-flap, or _pleuron_. On the upper side of the abdomen the terga of the somites overlap, the front part of each being pushed under the tergum in front when the abdomen is straightened, and only exposed to view when the abdomen is bent. Below, the sternum of each somite is seen to be only a narrow bar, connected with those in front and behind by soft membrane, and there is no overlapping. At the sides the somites are connected together by hinge-joints, which allow them to move only in a vertical plane. Thus the abdomen can be straightened out or bent downwards and forwards, but cannot be moved from side to side. In life the Lobster can swim backwards through the water by vigorously flapping the abdomen.

[Ill.u.s.tration: FIG. 2--ONE OF THE ABDOMINAL SOMITES OF THE LOBSTER, WITH ITS APPENDAGES, SEPARATED AND VIEWED FROM IN FRONT. (From British Museum Guide.)]

The carapace which covers the upper side of the head and thorax is not formed, as might be thought, simply by the terga of the somites becoming soldered together. This is shown by a comparison with certain shrimp-like Crustacea (_Mysidacea_) in which the carapace arises, like a fold of the skin, from the hinder edge of the head, and envelops, like a loose jacket, the distinctly segmented thorax. In the Lobster this fold has become adherent to the thoracic somites down the middle of the back, but at the sides it hangs free, enclosing on each side a cavity within which lie the gills.

It seems at first sight strange to include in the same category as "limbs" or "appendages" organs which differ so much in form and function as do the swimmerets, the walking legs, the jaws, and the antennae.

Nevertheless it can easily be demonstrated that all of them are constructed on the same general plan, and arise in the embryo from rudiments which are, for the most part, exactly alike. This is expressed in technical language by saying that the appendages of the whole series are _h.o.m.ologous_ with one another. A full discussion of this interesting fact would require more s.p.a.ce than can be devoted to it here, but a few examples may be given to ill.u.s.trate what is meant by the "serial h.o.m.ology" of the appendages in Crustacea.

If one of the swimmerets be detached from the third abdominal somite, it will be seen (Fig. 2) to consist of a stalk, known as the _protopodite_, bearing two branches, of which that on the outer side is called the _exopodite_, and that on the inner side the _endopodite_. The protopodite consists of two segments, the first very short, and the second much longer. It can easily be seen that the side-plates of the tail-fan (the middle plate, as already mentioned, is the telson) are simply the swimmerets of the sixth abdominal somite. They are much larger than the other swimmerets, and have the endopodite and exopodite broadened out into large plates; while the protopodite is very short, and not divided into segments.

[Ill.u.s.tration: FIG. 3--THIRD MAXILLIPED OF LOBSTER. (From British Museum Guide.)]

If now the third maxilliped (Fig. 3) be examined, it will be found that, like the swimmeret, it consists essentially of two branches springing from a stalk of two segments. The exopodite, however, is much smaller than the endopodite, and it ends in a flexible lash made up of many small segments. The endopodite forms the main part of the limb, and has five segments, so that, with the two segments of the protopodite, there are _seven_ segments in the main axis of the limb; the second and third segments are partly soldered together, but the line of union can be plainly seen. Attached to the outer side of the first segment is a membranous plate, known as the _epipodite_, on which is inserted, near its base, a brush-like structure, which is one of the gills. In the natural position the epipodite and its gill lie in the gill chamber, hidden from view by the side-flap of the carapace.

[Ill.u.s.tration: FIG. 4--WALKING LEGS OF LOBSTER

A, Of first pair; B, of third pair]

The legs (Fig. 4) can, without difficulty, be seen to consist each of seven segments like those of the maxillipeds, but there is no exopodite.

In the young Lobster, when just hatched from the egg, however, each of the legs has a large exopodite like that of the third maxilliped. These exopodites, which are used in swimming, are afterwards lost as the animal grows; but their presence in the young is interesting as confirming the conclusion that the legs, like the maxillipeds, are built on the same plan as the swimmerets. The large claws, and also the first and second pairs of walking legs, end in pincers, or _chelae_, the penultimate segment projecting in a thumb-like process against which the last segment works. Each leg, except those of the last pair, has on its first segment an exopodite with a gill like those of the maxilliped.

[Ill.u.s.tration: FIG. 5--APPENDAGES OF LOBSTER IN FRONT OF THIRD MAXILLIPED

A, Eye-stalk; B, antennule; C, antenna (the flagellum is cut short); D, mandible; E, maxillula; F, maxilla; G, first maxilliped; H, second maxilliped. _en_, Endopodite; _ep_, epipodite; _ex_, exopodite; _gn_, gnathobases, or jaw-plates; _p_, palp of mandible; _sc_, scaphognathite]

Following the series of appendages forwards from the third maxilliped (Fig. 5), it is easy to trace the gradual reduction of the endopodite and exopodite; while the two segments of the protopodite become flattened and broadened inwards to form the jaw-plates. The mandibles (Fig. 5, D), which are the chief organs of mastication, consist mainly of the much enlarged basal segment of the protopodite, with a strongly toothed inner edge, where it works against its fellow of the opposite side; and the rest of the limb is reduced to a small sensory "palp,"

which represents the second segment of the protopodite and the endopodite.

The antennae (Fig. 5, C) can be shown, without difficulty, to conform to the same plan of structure as the other appendages. The two segments of the protopodite are short, but distinct; the endopodite forms the long lash, or _flagellum_, composed of very numerous small segments; the exopodite is reduced to a small movable scale or spine.

The antennules (Fig. 5, B) seem at first sight to present the two-branched type of structure in its simplest form; but there is considerable doubt as to whether the two lashes which each bears on a three-segmented stalk are really equivalent to the endopodite and exopodite.

The movable stalks which carry the eyes (Fig. 5, A) have been considered by some to belong to the series of the appendages, and to be, in fact, modified limbs. If this be the case, we have here the greatest simplification which the limb undergoes in the Lobster, for each eye-stalk consists only of two segments: the first small and incompletely formed, the second in the form of a short cylinder, having the eye at its end. There are, however, reasons for doubting whether the eye-stalks are really appendages.

The hard outer covering of the Lobster not only protects and gives support to the internal organs, but also affords points of attachment for the muscles by means of which the animal moves. In other words, it plays the part of a skeleton; but since, unlike the skeleton of vertebrate animals, it is _outside_ instead of _inside_ the soft parts of the body, it is known as an _exoskeleton_. Closer examination shows that this outer covering is really continuous over the whole of the body and limbs, but is thin and soft at the joints, allowing the parts to move one upon another. It is composed of a horn-like substance known as _chitin_, which, except at the joints, is hardened by the deposition in it of carbonate and other salts of lime.