Chapter XIII: Marine Arthropods (1)

The sub-kingdom _Arthropoda_ contains a vast assemblage of animals, all of which, as the name implies, possess jointed appendages. Their bodies are covered with a skin that is hardened by a horny substance (_chitin_), and frequently, also, by the deposit of carbonate of lime.

The body of Arthropods is made up of a chain of segments, all of which are built up on one common pattern, and each one is surrounded by a ring of the hardened skin or exo-skeleton that gives attachment to a pair of appendages. Commonly, however, two or more of the segments become fused together, being covered by a continuous plate or shield, in which the boundaries of the rings are almost or completely obliterated; but in such cases the appendages they bear always remain distinct, so that the true number of segments is always apparent. The skin between those segments that are not so fused together remains soft and flexible, thus allowing the body to be freely bent.

The appendages exhibit a great variety of structure, and are as varied in their functions. Some are used as feelers, and others as jaws for seizing or masticating food. Some are developed into powerful seizing organs for purposes of defence or attack, some into paddles for swimming, while others are legs adapted for walking.

All these appendages are made up of segments, each of which, like those of the body itself, is surrounded by a ring of hardened skin, and connected with its neighbours by a flexible integument that allows perfect freedom of movement; while within are the muscles, often very powerful, by which the appendage is moved.

In the arthropods we have a sub-kingdom of highly organised animals, with distinct, and often very complicated, systems of organs for digestion, circulation, and respiration; and the nervous system consists of a well-developed chain of ganglia, connected by nerve cords, and from which nerve fibres are distributed to the various parts of the body. It should be noted, however, that some members of the group have degenerated into parasites, and in these, as with all such degraded creatures, many of the organs have retrogressed to such an extent that they are quite functionless, or have even disappeared entirely. These parasitic forms, when very young, are really highly organised creatures, not unlike the young of their industrious and more noble relatives; but, as the natural result of their degraded mode of living, in which they find no use for their organs of locomotion, digestion, circulation and respiration, these eventually disappear, with the result that the organs of reproduction predominate to such an extent that they often fill the greater part of the cavity of the body.

It should be noted, too, that the sense organs of arthropods are well developed, most of them being supplied with complex eyes, hearing organs, and highly sensitive feelers.

This sub-kingdom consists of four classes--the _Crustacea_, including lobsters, crabs, shrimps, prawns, &c.; _Arachnoidea_, containing spiders, mites, and scorpions; _Myriopoda_--centipedes and millepedes; and _Insecta_.

The first of these classes consists mainly of marine animals, and will therefore occupy much of our attention, but the members of the other three are mostly terrestrial and aërial creatures that do not fall within the scope of this work, except in the case of a few species that are more or less decidedly marine in their tendencies. The aquatic members are generally provided with well-formed gills by means of which they are enabled to extract the dissolved oxygen from the water in which they live, while those of terrestrial and aërial habits breathe by means of a system of tracheæ or air-tubes that are open to the air and supply branches to all parts of the body.

The _Crustaceans_ are mostly gill-breathers, though some of the aquatic species have no special organs for respiration, but obtain the oxygen necessary for respiration by absorption through their thin, soft skin, while the terrestrial species breathe by means of tracheæ, as we have just observed.

Most of them are covered with a calcified skin, as in the case of crabs and lobsters; but many are protected with a chitinous or horny covering such as we observe in shrimps and prawns. In either instance the hardened integument constitutes what is known as the _exo-skeleton_. None of the crustaceans have an internal skeleton of any kind, though some of the inner parts are supported by extensions of the hard skin that penetrate into the body.

It will be readily understood from the nature of the exo-skeleton of the crustacean, and especially of the more or less rigid calcareous covering of the crab and the lobster, that a uniform growth of the body is absolutely impossible, and, in fact, that an increase in size cannot take place without an occasional casting of the hard coat of mail. Hence we find most crustaceans throwing off their coverings at intervals, and growing by fits and starts during the periods between the ‘moultings’ and the hardening of the newly exposed skin.

When a crab or a lobster is about to undergo the process of moulting, it retires to a secluded niche in the rock, where it is not so easily found by its numerous enemies--a necessary precaution, since the creature in its soft or unarmoured condition is eagerly devoured by fishes and other marine animals--and there awaits the first stage of the ordeal. Presently the skin splits; and, after a time, the crustacean succeeds in extricating itself from its shell, which is cast off in a perfect condition, every joint being entire, even to the coverings of the antennæ, the stalked eyes, and other delicate appendages. And not only this, for the portions of the shell that penetrate inward into the body are also discarded, as well as the linings of the stomach and the gills; and these cast-off coats of crabs and lobsters--especially the former--may often be found in the most perfect condition on the sea shore, being washed up without injury on the sandy beach, or found in the very niche in which the creature changed its attire.

If one examines the powerful pincers of a crab or lobster, a thin plate of considerable size will be seen to extend within from the movable ‘jaw’ to give attachment to the muscles by which it is moved, and it seems impossible that this can be removed with the cast skin without considerable injury to the new claw that is already formed, though as yet in a soft condition, within the old and hard one. But it has been observed that this plate actually cuts through the new claw, and that the claw thus divided almost immediately closes up and unites again.

The moulting process being over, the crustacean’s body extends itself within the new, yielding skin; and, the latter becoming gradually hard by the deposition of carbonate of lime, the creature is able, after a period of rest, to roam at large again, without much fear of injury, until the time for the next moulting has arrived.

Those who have made but a slight acquaintance with the common crustaceans of our shores must have noted the frequency with which imperfect specimens occur--specimens with missing appendages, or with a well-formed limb on one side of the body opposed to a puny and almost useless fellow on the opposite side. As to the loss of appendages, this matter will be readily understood by those who have watched crustaceans, and especially crabs and lobsters, in their native element, so often do these pugnacious creatures become engaged in furious broils with their neighbours. And, when we are at work at the collection of various species on the sea shore, how often do we find that a creature escapes from our grip by leaving us in possession of a severed limb, while the owner retreats rapidly among the stones and weeds apparently none the worse for its trifling loss! This is, in fact, a very common method of securing its escape from an enemy; and it appears that many crustaceans have the power of thus rendering a seized limb so brittle that it may be snapped off with the greatest of ease.

We have spoken of the loss thus sustained as a trifling one; and so it is, for crustaceans have the faculty of reproducing lost appendages; and though the loss may be one of considerable inconvenience at first, a new limb eventually appears in the place of each one so willingly discarded.

When such mutilations occur, it will be observed that the severed limb invariably breaks away at the end of the first or basal joint--a point where the bloodvessels are so narrow and contractile that but little loss of blood takes place when the rupture is made--and it has been said that the animal would soon bleed to death if the fracture were to take place at any other point. As it is, the wound soon heals, but no trace of a new limb is to be seen, at least without dissection, until the time of the next moult. The part is developing, however, beneath the cover of the basal joint; and when the moulting period arrives, the new limb, still very small, is exposed to view. It then rapidly enlarges, though not to anything like its proper size, and its surrounding skin becomes hardened by the deposit of the calcareous secretion simultaneously with that of the rest of the body. Further enlargements of the new appendage take place at subsequent moults, with the final result that it is but slightly inferior to its fellow either in size or in power.

The eye of a crustacean is a very complicated structure, commonly described as a compound eye. It consists of a large number of conical, radiating, crystalline rods, collected together into a mass that presents a convex outer surface. This surface is covered with a transparent layer of chitin which naturally presents a more or less distinct netted appearance, the bases of the rods being in contact with its inner surface, and visible through it. Each rod is surrounded by a layer of pigment that prevents light from passing from one to another, and the optic nerve passing into the base of the compound structure sends a sensitive filament into each one.

In many crustaceans this compound eye is situated on the end of a movable stalk that generally allows it to be protruded or drawn under cover as occasion requires, but in others the organ does not project beyond the general surface of the body. Thus we hear of the animals of this class being divided into the _stalk-eyed_ and the _sessile-eyed_ groups; the former being represented by crabs, lobsters, shrimps, &c.; and the latter by sandhoppers and sandborers.

Crustaceans undergo metamorphoses while very young, the body being altered considerably in form at several successive moults. Some, in their earliest stage, consist of a little oval body that shows no signs of a division into segments. It swims about by means of three pairs of appendages, and has only one eye. Others start life with four pairs of limbs, attached to the front portion of the body, a segmented abdomen, as yet perfectly limbless, and a pair of compound eyes. Then as the successive moultings take place, new segments and new appendages are developed, until, at last, the form of the adult is assumed. The accompanying illustration shows four stages in the development of the Common Shore Crab.

The lowest division of the crustaceans contains the _Cirripedia_ or Curl-footed crustaceans, which includes the Barnacles that are so frequently seen attached to the bottom of ships and of floating timber, and the Acorn Barnacles, the conical shells of which often completely cover large masses of rock on our shores.

For some time naturalists could not agree as to the proper place of these animals in the scale of life, but the matter was finally settled when some minute creatures only about a twelfth of an inch in length, and closely resembling the early stages of certain crustaceans, were seen to undergo metamorphoses, and finally develop into acorn barnacles. Their position in the animal kingdom was thus determined by their early stages; but these, instead of changing into a segmented and highly organised creature like the typical crustacean, lose some of their appendages, cease to be free-moving animals, and attach themselves to floating bodies by which they are carried about. Thus they are enabled to find the food they can no longer seek without such aid. In their young state they possess not only the means of freely moving in search of their food, but have organs of vision to aid them in the capture of their prey. As they grow, however, the foremost appendages are transformed into a sucking-disc, and the eyes, no longer necessary, disappear. It will thus be seen that the degenerated adult--the product of a _retrograde development_--is attached by what was originally the front of its body, while the abdomen is undeveloped, and the thorax, with its appendages, forms the summit of the free extremity.

Some of the Cirripedes attach themselves to the bodies of whales and other marine animals. The majority of these are pseudo-parasites--creatures that live on the bodies of other animals, but do not derive their food at the expense of their hosts; others, however, are true parasites, subsisting on the nourishing juices they extract from the animals to which they are attached.

The Acorn Barnacles, so numerous on our shores, are good types of the _Cirripedia_, and they are so easily kept alive in the indoor aquarium that their interesting movements may be well observed. A cluster of these animals may be obtained by chipping off a piece of the rock on which they grow; or, instead of this, a few minutes’ searching on a rocky coast at low tide will certainly provide us with a stone of suitable size, or the shell of a mollusc, on which the creatures have found a home.

Place them in the indoor aquarium, or in any shallow vessel containing just sufficient sea-water to cover them, and carry out your observations with the aid of a hand lens. They will soon open the inner cone of their many-valved shell, and slowly protrude six pairs of gracefully curved and delicately-feathered appendages which, as previously stated, are attached to the thoracic portion of the body. Then, with a much more rapid movement, the appendages will be withdrawn, and the shell closed. These alternate movements are continued incessantly, and are the means by which the animals provide themselves with both food and air. The reader should also obtain some specimens of the larger species for the examination of the shell, the structure of which is interesting and, of course, peculiar to this order.

In general structure and habits Barnacles are very similar to the acorn barnacles, except that the body is supported on a tough stalk, which, as we have already stated, is the modified anterior portion of the animal. These animals also may be easily kept alive and examined in the indoor aquarium. They are not creatures of the sea shore, but may often be obtained on masses of timber that have been washed ashore, or from the bottoms of ships that have been placed in the dry dock for repairs.

Another order of the crustaceans--the _Copepoda_, or oar-footed group--is so called on account of the bristled feet that are employed after the manner of oars when the creatures are swimming.

These Copepods are small animals, so small indeed that the compound microscope is generally necessary merely for the examination of their external characters. Many species inhabit fresh water, and the study of the group is more commonly pursued by the investigator of fresh-water pond life than by the sea-side naturalist. However, marine species are abundant, and may be captured in the open water or in rock pools by means of a muslin net. As with the last order, some degenerate from the comparatively complicated free-swimming and eyed larval state to blind and limbless parasites that feed on the bodies of fishes and are known as fish lice.

The body of the typical copepod is distinctly segmented, and the head and thorax are both enclosed in a hardened buckler. It has two pairs of antennæ, two pairs of foot jaws by which it captures its prey, and four or five pairs of bristled feet for swimming. The jointed abdomen has also a tuft of bristles at its extremity. The annexed illustration represents some marine species, and will serve to show the general features of the order.

The sea-side naturalist, intent on the collection of small life, may possibly meet with representatives of two other orders of crustaceans--the _Ostracoda_ or shelled crustaceans, the bodies of which are enclosed in a bivalve, hinged shell; and the _Branchiopoda_, so called because the branchiæ or gills are attached to the feet.

The Ostracodes have two or three pairs of feet which subserve locomotion, but are not adapted for swimming; and two pairs of antennæ, one of which assists in locomotion. The mouth is provided with organs of mastication, the branchiæ are attached to the hind jaws, and the animals have but one eye. Some of these crustaceans inhabit deep water only, while others live in sand between the tide-marks; but several species, belonging chiefly to the genus _Cythere_, abound in rock pools, where they may be readily obtained by scraping the confervæ and corallines with a small muslin net.

The branchiopods are free swimmers, and are protected by a buckler-like envelope. Most of them are inhabitants of fresh water, and are popularly known as water fleas. We have figured one marine species, belonging to the genus _Evadne_, which has a colourless body, and a single conspicuous black eye, and is interesting as being the food of the herring.

The four orders of crustaceans that have been briefly described belong to the division _Entomostraca_, which signifies ‘shelled insects.’ This term is not a happy one when judged from the standpoint of our present knowledge of animal life, but it must be remembered that, at the time it was applied (1785), spiders and crustaceans were all included in the same class as the insects; and this is hardly surprising when we observe the close relationship of these animals, as shown in their segmented bodies and jointed appendages; for, as we have already shown, the lowly organised parasitic crustaceans which, in the adult state, lose most of their appendages and cease to be distinctly segmented, are more or less insect-like in their larval and free-swimming stage.

All the other crustaceans are included under the term _Malacostraca_, or soft shelled, since, although many of them are protected by an exo-skeleton that is hardened by the deposit of carbonate of lime, yet, generally speaking, their coverings are softer than those of the molluscs; and therefore the term _Malacostraca_ was originally applied by Aristotle in order to distinguish them from the animals that are covered by harder and thicker shells.

This division of the crustaceans contains wood lice, sandhoppers, lobsters, shrimps, crabs, &c., and consists of two main groups--the Sessile-eyed (_Edriophthalmata_) and the Stalk-eyed (_Podophthalmata_) crustaceans.

We shall now consider the Sessile-eyed group, dealing first with the order _Isopoda_ or equal legged, and then the _Amphipoda_, which have appendages adapted both for walking and swimming.

The general nature of an Isopod may be readily understood by the examination of the common woodlouse that abounds in gardens and damp places almost everywhere, and the reader will probably remember having seen similar creatures crawling over the rocks on the sea shore.

The body is generally depressed or flattened, but convex above, and is composed of seven segments, each segment bearing a pair of legs which terminate in a pointed claw, while the posterior appendages are modified into flat, leaf-like organs of respiration.

When engaged in ‘shrimping’ one frequently meets with shrimps or prawns that are disfigured by a tumourous swelling on the side of the body, and if the swelling be opened a little parasite will be dislodged. This parasite is an Isopod (_Bopyrus_), the appendages of which are imperfectly developed. The female is very much larger than the male, and, as is usual with parasitic creatures, the greater part of the body-cavity is occupied by the well-developed organs of reproduction.

There are several other parasitic isopods, some of which live on the bodies of fishes, and are popularly known as fish-lice, but these are not so likely to come in the way of the sea-side naturalist as the more typical forms that crawl about on the rocks and among the weeds of the coast. A few of the latter are shown in the accompanying illustration, including the Sea Pill-ball (_Nesæa bidentata_), common on the rocky coasts of the south-west, and distinguished by the two sharp projections on the last segment; the Serrated Pill-ball (_Sphæroma serratum_), very common on most rocky shores, and characterised by the fine sawlike teeth on the outer edge of the outer plates of the ‘tail’; the Great Sea-slater (_Ligia oceanica_), also an abundant species; the Spotted Hog Louse (_Oniscoda maculosa_) that lives among the tufted sea weeds; and the Boring Pill-ball (_Limnoria lignorum_) that bores into the woodwork of piers and jetties, often doing considerable damage.

The above and other isopods feed on various animal and vegetable substances, some species being quite omnivorous in habit. Most of them are eagerly devoured by birds and fishes.

The Amphipods, six species of which are shown in the above illustration, include the Sandhoppers or Beach Fleas, so numerous on our coasts that it is almost impossible to go any distance without making their acquaintance. They are invaluable as scavengers, as they rapidly devour decaying sea weeds, and will speedily reduce the body of any animal washed on the beach to a clean skeleton. Although they are all small creatures, they make up in numbers for any deficiency in size; and though devoured in enormous quantities by the various shore birds, they multiply so prodigiously that they are never lacking wherever there is decomposing organic matter to be consumed.

The bodies of these animals are usually flattened from side to side, very distinctly segmented, and have a well-developed abdomen. The head is furnished with two pairs of antennæ and a pair of sessile eyes, though some species possess only one pair of antennæ, while others have four eyes. The limbs of the thorax are used either for walking or for swimming, and give attachment to the gills. The abdomen has generally six pairs of appendages, the foremost three pairs of which are usually small, and employed in swimming, while the others are stronger and directed backwards, and are often adapted for jumping.

It is very interesting to observe the habits of the Sandhoppers and other Amphipods both on the sandy beach and in the water, and the student will find that certain species burrow into the sand with considerable agility, and live principally at the extreme high-water mark, where they feed on the organic matter washed in by the breakers at each high tide, while others dwell almost exclusively in the water, among weeds and stones, and should be searched for at low water. The latter may be kept alive for some time in the aquarium providing they are the only occupants, but a little experience will show that these and all other Amphipods are readily devoured by many marine creatures, and consequently they are of real value to the aquarium keeper as food for other animals.

We now come to the Stalk-eyed Crustaceans (_Podophthalmata_), which contain those members of the class most generally known, such as crabs, lobsters, shrimps, and prawns. In these the eyes are mounted on movable pedicels, the head and thorax are generally covered by a large shield called the carapace, and the appendages are adapted partly for seizing and masticating, and partly for locomotion.

The group includes two orders--the _Stomapoda_ or Mouth-footed crustaceans, so called because some of the limbs are crowded round the region of the mouth; and the _Decapoda_, or Ten-footed crustaceans.

The Stomapods, though very abundant in tropical seas, are not often met with on our own shores. However, since a few interesting species are inhabitants of our seas we will briefly describe the distinguishing characteristics of the group.

We have just mentioned the fact that the head and thorax of a decapod is usually covered by a large shield--the carapace. Now, the general character of this carapace may be seen at once in either the shrimp or the lobster. In these animals the segments that form the head and the thorax are all fused together, and are completely covered by the protective buckler of hardened skin; but in the Stomapoda the carapace is much smaller in proportion, and a few of the segments of the thorax, instead of being fused into the general mass of the _cephalo-thorax_, are quite distinct from it. The abdomen, also, is large and strongly formed in these animals. Five pairs of the thoracic limbs are directed forwards, and are adapted both for catching food and for climbing, while others are used in walking. The limbs of the abdomen generally number six pairs, of which the first five bear feathery gills.

Two species of Mantis Shrimps, one of which is represented in fig. 201, have been found off the south and south-west coasts, but these are not likely to be seen on the shore, since they inhabit deep water. Allied to these, and sometimes included with the Stomapods, are the Opossum shrimps, so called because the females of some species carry their eggs in a kind of pouch, thus reminding us of the marsupial quadrupeds of the same name. They are of very slender build compared with the mantis shrimps, and differ from them in that the carapace completely covers the thorax; but though this is the case, the fusion of the thoracic segments is not complete, since the posterior ones have still a certain amount of freedom of movement. Some species of opossum shrimps are abundant in the rock pools of our coasts, particularly in the south-west, but their bodies being often so transparent as to be almost invisible, they are consequently easily overlooked. Their general appearance may be gathered from our illustration of _Mysis chamæleon_, which is probably the most common species inhabiting our coast.

The highest crustaceans--the Decapods--are divided into two sub-orders--the _Macrura_, or Great-tailed, including lobsters, shrimps, &c.; and the _Brachyura_ (Short-tailed), containing the crabs; but the number of British species is so large that it is impossible to give, in our limited space, a detailed description of all the commonest even. All we can do is to note a few of the more interesting features of certain species, to introduce such illustrations as will enable the young naturalist to identify a number of the commoner ones, and to give the general characteristics of the main divisions so that the student may be able to classify his specimens intelligently.

In the _Macrura_, as with other divisions of the crustaceans, we meet with very interesting modifications of the appendages, adapted to quite a variety of uses; and if the reader is unacquainted with these adaptations of structure to habit he cannot do better than secure a lobster or crayfish for study. It will be observed that the body may be divided into two main portions--the _cephalothorax_, consisting of head and thorax combined, and the _abdomen_. The former is composed of fourteen segments, so thoroughly fused together that they are denoted only by the fourteen pairs of appendages to which they give attachment, while the calcified skin forms one continuous shield surrounding the whole. The abdomen, on the other hand, consists of six distinct segments, each of which is surrounded by its own ring of the hardened integument, and is connected with its neighbours by means of a portion of uncalcified skin that renders the whole very flexible. A groove in the front portion of the great shield (_carapace_) marks the division between the head and the thorax, the former composed of six, and the latter of eight united segments.

The calcareous covering of each segment consists of an upper portion, called the _tergum_, and a lower, named the _sternum_, united at the sides; the sternal portion of the cephalothorax, which gives attachment to the walking limbs, is a most complicated and beautifully formed structure.

The six pairs of appendages belonging to the head are easily made out with a little care. The first are the jointed _eye-stalks_ that bear the compound eyes previously described; and these are followed by two pairs of _antennæ_, or feelers, the first being shorter and double, while the second are very long. The former contain the organs of hearing. Then, in front of the mouth, and completely hiding it, are a pair of strong _mandibles_ or jaws that move horizontally, and the two pairs of _maxillæ_ that are also employed in reducing the food.

Following these, but belonging to the thorax, are three pairs of appendages that are known as foot-jaws; for, although they assist the preceding organs in breaking up the food, they bear a resemblance in some respects to the longer limbs behind them. Of the latter there are five pairs (hence the term _decapoda_), the first being a very powerful pair of seizers or pincers, and the remaining four, which are well adapted for walking, terminating in either double or single claws.

All the appendages above mentioned are not only attached to the body by movable joints, but are themselves made up of jointed parts, sometimes a considerable number, each of which, like the segments of the body itself, is surrounded by a ring of hardened skin, and connected with those above and below it by a portion of soft and flexible skin.

Lastly, beneath the abdomen, are paired limbs called _swimmerets_, which are used as paddles, and probably assist the animal more or less in its progress through the water; but the principal organ of locomotion in the _macrura_ is undoubtedly the powerful muscular abdomen, aided by the broad and fanlike tail formed by the appendages of the last segment. To demonstrate this fact, put a live lobster, or even a shrimp, in a still rock pool, and threaten it from before, when it will rapidly retreat backwards by a series of powerful jerks, produced by suddenly doubling its abdomen forwards beneath its body.

In addition to the external characters above mentioned, there are many interesting features connected with the internal structure of the lobster that may be studied on making easy dissections. Thus, the gills, which are attached to the bases of the thoracic limbs, may be exposed by cutting away the side of the carapace, and at the same time we may discover the bailing organ by means of which a current of water is kept flowing forwards through the gill-cavity to keep up the necessary supply of oxygen for respiration. The removal of the upper portion of the carapace will expose the heart and some of the principal bloodvessels, and also the stomach with its powerful and complicated ‘gastric mill,’ formed by the hardening of portions of the wall of the latter organ for the purpose of crushing and masticating the food. Then, if these organs be carefully removed from above, together with the others we have not space to describe, and the powerful muscles that fill up the segments of the abdomen, the chain of ganglia and their connecting nerve cords that form the central part of the nervous system may be seen extending along the central portion of the body.

Several species of lobsters inhabit our seas, but they are generally to be found beyond the tide-marks, and are, therefore, not often caught by sea-side collectors without the aid of some kind of trap or the assistance of fishermen. The common lobster (_Homarus vulgaris_), however, is often left behind by the receding tide on our rocky coasts, and may be seen and caught if one knows where to look and how to capture.

On cautiously approaching a deep rock pool one may often see a lobster rapidly retreat in its usual backward fashion, and snugly house itself in a narrow chink from which it is impossible to remove it. And, when once surprised, it is not likely to show itself again as long as the intruder is in view.

If one remains perfectly still for a time, a pair of waving antennæ may be seen gradually protruding from the safe retreat; but, as soon as the stalked eyes have advanced sufficiently to detect the figure of a stranger, the lobster silently withdraws itself till quite out of sight.

Lobsters, usually of rather small size, may often be seen quite out of the water at low tide, in the narrow chinks of the rock, or under large stones, but it is no easy matter, as a rule, to get them out. It is of little use poking a stick into the entrance of their hiding-places, though occasionally they will grasp the stick so tenaciously with their forceps that they may be pulled within reach. You _may_ be able to haul them out by their long antennæ, but if you can find a second way into their home such that you can disturb them from behind you are pretty sure of your victim.

It will be unnecessary to describe other species of lobsters individually, but we have introduced figures of a few for identification. The Norway Lobster (_Nephrops norvegicus_) is often landed in large numbers by the fishermen of the east and south-east coasts and sold at a shilling or so a dozen under the name of Norway Prawns. They are pretty and interesting creatures, and may be easily kept alive in the indoor aquarium, where they may be fed on any kind of fish.

Fig. 209 represents the two allied creatures that may sometimes be dug out of the sandy beach, or from the mud in the estuary of a river. The one on the left is the mud-borer (_Gebia stellata_), which is of a dull yellowish colour, marked more or less distinctly by pinkish starlike spots--a feature that has suggested the specific name. The beak in front of the carapace is very prominent and spiny, and the long abdomen is narrower in front than in the middle. This creature hides in the holes that have been excavated by boring molluscs, and seems also to extend the cavities it inhabits by its own labours.

The other is very similar in general form, but has no spiny beak and the abdomen is much broader in the middle than at the base. It is also to be distinguished by the very unequal size of its front legs, one of which is much more developed than the other.

It is known as the mud-burrower (_Callianassa subterranea_), and is said to burrow very deeply into mud-banks, scooping out its retreat principally by means of the second and third pairs of legs. Although found at times between the tide-marks, its principal habitat is probably in the mud that is covered by deep water, for it is not uncommonly to be found in the stomachs of fishes that habitually feed in such localities.

Lobsters of all kinds, and, indeed, the marine crustaceans generally, are essentially the scavengers of the sea, for they are carrion-feeders, greedily devouring flesh in all stages of decomposition. Hence the value of their work on the sea shore is very considerable.

An examination of shrimps and prawns will at once show their close relationship with lobsters. The general build of their bodies is practically the same, and their appendages, though often different in form from the corresponding limbs of the lobster, will be seen to resemble them closely in arrangement and structure. The exo-skeletons of these creatures are, however, generally hardened by a horny substance (_chitin_) instead of a stony deposit of carbonate of lime.

The shrimps and prawns sold for food in our markets are very similar in appearance when alive, the leading distinguishing feature being, perhaps, the presence of a sharp, serrated beak projecting forward from the front portion of the carapace of the latter.

The reader is probably acquainted with the fact that the shrimps and prawns used as food have quite a different appearance when alive and in their native element to that displayed by the corresponding wares in the fishmonger’s shop--a fact that applies equally well to the edible crabs and lobsters. Most crustaceans change to a bright red colour when boiled, and, as stated in a previous chapter, the same result is produced by the action of strong spirit.

The Common Shrimp (_Crangon vulgaris_) is an exception, however, for it may be distinguished when boiled by its dull greyish brown colour. When alive this species is of a very pale greenish or greyish tint, lightly spotted with brown; and its habits are so interesting that it will well repay one to watch it either in the aquarium or in a rock pool. It frequents sandy coasts, and can hide itself very quickly by burying its body in the sand, using for this purpose both its legs and its antennæ.

The Prawn frequents rocky coasts, where it may often be obtained in large numbers by sweeping with a suitable net under the cover of weeds and stones. Its body is of an exceedingly pale greenish colour, and so transparent that it is quite inconspicuous when in the water. Prawns are turned to a rose-red colour by boiling, and they are captured in large numbers when young and sold as ‘red shrimps.’

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FIG. 211.--THE PRAWN (_Palæmon serratus_)
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In addition to the common species mentioned there are quite a number of shrimps and prawns to be found in our seas, but some of them inhabit deep water and are rarely to be found between the tide-marks. All, however, are eagerly devoured by fishes, and, on that account, are often to be obtained in good condition by examining the contents of the stomachs of freshly caught fishes. In fact, this mode of search for the smaller species of deep-sea life is not to be despised, for it is a means by which we can obtain specimens that are not often secured by the methods coming within the ordinary range of the amateur’s work.

It will be remembered that we spoke of the Decapods as consisting of two main groups--the Great-tailed (_Macrura_) and the Short-tailed (_Brachyura_). Frequently, however, we find the order divided into three sub-orders as follows:--

1. _Macrura_ (Great-tailed), 2. _Anomura_ (Peculiar-tailed),
3. _Brachyura_ (Short-tailed);

the first containing lobsters, shrimps, &c.; the third the typical crabs, such as the shore crab and the edible crab; while in the second are placed those species of crabs which have been regarded as intermediate in character. Thus, in the _Anomura_ we find decapods in which the abdomen, though not so well developed as in the _Macrura_, is either permanently extended or is capable of being extended and used for swimming as occasion requires. The hindmost legs, also, are not well developed and adapted for walking, but are employed only as organs of prehension; and, as is the case with the first sub-order, there are often two pairs of well-developed antennæ.

In this sub-order of ‘queer tails’ we find the Soldier or Hermit Crabs, and those flat-bodied crabs that live almost exclusively on the surface of stones, and are hence known as Stone Crabs; but as opinion now seems inclined against the formation of a special suborder for these creatures, we shall briefly deal with them as a first section of the _Brachyura_.

The Stone Crabs are extremely interesting creatures, and the observation of their habits, both in and out of the water, is particularly entertaining and instructive. One species--the Broad-Clawed Porcelain Crab (_Porcellana platycheles_), shown on Plate VI.--is very abundant on all our rocky coasts, and may be found in immense numbers near low-water mark.

Turn over some of the large encrusted stones that strew the beach among the rocks, and you are almost sure to find numbers of these little crabs clinging to the freshly exposed surface. A few of them may remain perfectly still, and exhibit no sign of surprise on their untimely exposure to the light; and these, on account of their small size, the closeness with which they apply their flattened bodies to the encrusted stone, and more than all to the protective colouring of their dingy bodies, which so closely resembles that of the surface to which they cling, may well be overlooked by the inexperienced collector. But the majority of them will immediately scamper away in their own peculiar fashion towards the edge of the stone, and rapidly make their way to what is now the under side. As they progress with a hasty, sliding movement they never for one moment loosen their firm hold on the rough surface of the stone, but keep both body and limbs in close contact with it, clinging hard by means of their pointed claws as well as by the numerous hairs and bristles with which their appendages are liberally fringed.

Attempt to pull one from its hold, or even take other than the gentlest means to arrest its progress, and you will probably find that it suddenly parts company with one of its broad claws in its endeavour to escape; and, unless some special precautions be taken to remove these crabs, it is possible that quite half the specimens taken will have been damaged in this way during their struggles to escape. If, however, you gently thrust the point of a penknife beneath the body, and then apply the thumb above, you may lift them from a stone without injury. Another plan is to press a frond of smooth sea weed as closely as possible to the surface of the stone in the front of the crabs, and then allow them to crawl on to it, or cause them to do so if necessary. The piece of weed, with crab or crabs attached, may then be bagged for future examination.

On turning over the Broad-clawed Crab its under surface will be seen to be perfectly smooth, with an appearance closely resembling that of white porcelain. Its foot-jaws, also, are proportionately large, and closely fringed with hairs; and the last pair of legs, which are very slender in build, are folded closely beneath the body. Further, the abdomen is wide, composed of six distinct movable segments, and terminating in a tail-fin composed of five fringed plates.

Drop the crab into water, and it will immediately extend its abdomen, which it will flap sharply under its body somewhat after the manner of lobsters and shrimps, and thus swim backward by a series of jerks as it sinks to the bottom. On reaching the bottom it instantly grasps the solid material, applies itself closely to the surface, and glides away into the nearest chink it can find.

As one observes the nature and movements of these interesting little crabs one cannot fail to see how beautifully their form and structure are adapted to their habits. They are peculiarly constructed for abode in narrow chinks and crannies, and for feeding on the small forms of life that inhabit such sheltered places. Their legs move in the plane of their flattened bodies, and as they glide among the confervæ and other low forms of life that encrust the stones of the beach they feel their way by, and are possibly also guided by the sense of smell located in, their long outer antennæ, while the close fringes of their claws and foot-jaws form admirable sweep-nets by means of which the little animals that form their food are swept towards the mouth.

We have other species of stone crabs, one or two of which resemble the last species, and belong to the same genus, but the others are very different in general appearance. The Northern Stone Crab (_Lithodes_), found principally on and off the coasts of Scotland and Ireland, has a spiny covering with a long beak. Another species--_Dromia vulgaris_--is somewhat similar in habit, though it can hardly be termed a stone crab, since it inhabits deep water, and apparently lives among the sponges, sea firs, and weeds that cover the bottom.

The remainder of the Peculiar-tailed Decapods belong to the Soldier or Hermit Crabs, and constitute the genus _Pagurus_.

Every one who has searched a few rock pools will have seen the familiar Hermits, and will probably have been interested in their varied antics. First you observe the shell of a mollusc--a Trochus, Periwinkle, or a Whelk--travelling at an abnormal rate for a member of its class. You approach closely to make an inquiry into the matter, when the motion suddenly ceases, and the shell instantly drops into position with its mouth close to the surface below. If left undisturbed for only a short time, the rapid and somewhat jerky motion is resumed, only to cease as suddenly as before as soon as the inhabitant is again threatened.

On examining the shell we find that it is the home of a species of crab, and that the animal within it is completely hidden with the exception of its head, stalked eyes and long, slender antennæ, one very large claw, and a few walking legs.

To remove the creature from its home is no easy matter as a rule. To pull it out by means of its legs or its antennæ would probably be to sever some portion of its body; but if you thrust the creature, shell and all, among the spreading tentacles of a large anemone, it will at once grasp the peril of the situation; and, if the shell has already been secured by the clinging petals of this dangerous marine flower, the hermit will speedily quit its home and endeavour to rush from the many snares in order to secure its freedom. Or, it not infrequently happens that the occupied shell is one that has withstood many a storm, but not without the loss of the apex of its cone. In this case the insertion of a very flexible fibre into the opening thus made will cause the hermit to leave its home in the possession of the enemy.

Having, by some means or other, managed to drive the crab from its shell, we place it in a shallow rock pool, or in a vessel of sea water, and observe the chief features of its structure.

The first thing that strikes one is the absence of a calcified skin on the extended abdomen, which is so soft that, remembering with what eagerness fishes will attack and devour crabs of all kinds, we can at once understand the necessity of such a home as the creature selects. Again, we observe the presence of appendages at the tip of the abdomen by means of which the crab is enabled to hold itself securely in the shell. Also, when we note the general form of the armoured portion of the body, and the position of the soft-skinned abdomen, we can see how well adapted the whole is to fit snugly into the spiral shell of a whelk or winkle.

We also observe that one of the pincers is much larger than the other, and the value of such an arrangement may be estimated when we see the animal at home. The smaller claw, together with the other appendages used for walking or prehension, can be retracted within the shell, but the large claw, which constitutes a formidable weapon of attack and defence, is not only in such a position as to be ready for immediate use; but, lying as it does in front of the body, with other portions hidden more or less behind it, it serves the purpose of a shield when the animal retires.

If we place a homeless hermit crab in a rock pool, the behaviour of the creature immediately suggests a feeling of uneasiness--a sense of danger--for it moves about in a very erratic fashion that is quite different from the straightforward and deliberate action of the same animal when properly protected; and very amusing results may be obtained by making it the subject of a few harmless experiments. For instance, drop down before it an empty whelk-shell that is much too large to properly accommodate its body. It will immediately approach the untenanted house, search and probe it well with its antennæ and other appendages, and then, finding it uninhabited, and having no apartment of more suitable size at hand, will abruptly gives its body a turn and hastily thrust itself backwards into it.

If at the time of this experiment the advancing tide disturbs the water of the pool, the result is somewhat ludicrous, for the shell, too cumbersome to be controlled by the creature within, is, regardless of its attempts to maintain a normal position, turned over and over as each wave advances and retreats.

Again, supposing the shell supplied to be too small for the intended occupant, it will, after the usual examination of the interior, thrust its soft abdomen as far in as possible, and make the best of the unsatisfactory circumstances until a more suitable home can be found. And if, at this distressing period, we drop before it a shell of just the right size--the one from which the creature was originally expelled for instance, it is astonishing how quickly the change of houses will be accomplished. After a brief examination of the shell with the object of determining whether all is right within, during which the crab continues to avail itself of the imperfect accommodation afforded by the previous shell, it rapidly extracts its body from the one and thrusts itself backwards into the other. Its normal habits are at once resumed, all its movements being now suggestive of confidence and contentment.

We have already referred (p. 153) to the fact that a large anemone (_Sagartia parasitica_) is commonly found attached to a whelk shell, which at the same time forms the home of the hermit crab, and (p. 44) that a marine worm (_Nereis_) is also a common associate of the hermit, taking up its abode in the interior of the same shell; and we also briefly discussed the mutual advantage of such an arrangement to the parties concerned. These triple combinations are not so frequently met with on the shore between the tide-marks, but are dredged in considerable numbers by the trawler; and the reader will find it repay him to secure one in order that he may be able to watch the interesting habits of the associates. The movements of the hermit crab are always pleasing, particularly the manner in which it seizes and manipulates its food; and still more so is the occasional appearance of the head of the worm, always in exactly the same place, for the purpose of deliberately stealing the food from the very jaws of the crab.

Hermit crabs are easily kept in captivity, and may be fed on any kind of animal food, but care should be taken not to allow an excess of food to remain in the water and render it putrid by decomposition. As long as the crabs are active and remain within their shells you may assume that the conditions are favourable; but when they become sluggish in their movements, and leave their homes, the sanitary condition of the aquarium should be regarded with suspicion; for hermit crabs, like many of the marine tube worms, generally quit their homes when the conditions are unfavourable, as if they preferred to die outside.

The Common Hermit Crab (_Pagurus Bernhardus_), also known as the Soldier Crab, on account of its very pugnacious habits, is common almost everywhere on our coasts, and may be distinguished by the numerous little tubercles on the claws and on the upper edge of the front legs; and there are several other species, belonging to the same genus, distributed more or less locally on the various shores. All are similar in general structure and habits, the various species being identified principally by means of their colour, the variations in the form of the appendages, and the general character--smooth, tubercular, spiny, &c.--of the exo-skeleton. One species, found in the sandy bays of Cornwall, burrows rapidly in the sand.

Coming now to the true crabs--the _Brachyura_, or Short-tailed crustaceans, as sometimes distinguished from the _Anomura_--we find quite a variety of interesting creatures, many species of which are always within the reach of the collector at work between the tide-marks. In all these the abdomen is only slightly developed, and is never used in swimming, being permanently folded beneath the thorax. This portion of the body, however, is usually very distinctly segmented, and if it be lifted from its position it will be found that some of the segments bear appendages corresponding with the swimmerets of the lobster. It is also wider in the female than in the male, and crabs of the former sex may often be found during the summer with the abdomen more or less depressed, and the space beneath it quite filled with eggs.

The upper surface of the carapace of crabs is often very distinctly grooved, and it is interesting to note that these features of the exo-skeleton are not merely of external significance, for they usually correspond in position with various internal structures, some of them denoting the areas of the insertions of important muscles, and others enclosing the regions of certain of the internal organs.