The Life of Crustacea Part 4

The third section of the Reptantia, the _Anomura_, comprises forms in which the abdomen is variously modified, being either bent upon itself or, if extended, more or less soft and feebly armoured. The last pair of legs are commonly reduced in size, and not used in walking. The members of the four tribes composing the section differ widely in their general appearance.

The _Galatheidea_ (Plate VI.) are small, flattened, lobster-like animals which have the abdomen bent under the body. In one family (_Porcellanidae_) the animals have quite the appearance of little Crabs (see Fig. 41, p. 113), but they may be distinguished from the true Crabs (Brachyura) by the fact that there are only three pairs of walking legs behind the great chelae, the last pair of legs being very small and carried folded up at the sides of the body, or even within the gill chambers.

[Ill.u.s.tration: _PLATE VI_

_Munida rugosa._ BRITISH. (REDUCED)]

The _Thala.s.sinidea_ are small lobster-like animals which burrow in sand and mud, and have generally a more or less soft abdomen (see Fig. 38, p.

103).

[Ill.u.s.tration: _PLATE VII_

THE COMMON HERMIT-CRAB, _Eupagurus bernhardus_, IN THE Sh.e.l.l OF A WHELK (REDUCED)

(_From Brit. Mus. Guide_)]

The tribe _Paguridea_ includes the Hermit Crabs (_Paguridae_) and their allies. The typical Hermit Crabs (Plate VII.), which are familiar objects in seaside rock-pools, live in the empty sh.e.l.ls of Whelks and other Gasteropod Molluscs, which they carry about with them as portable shelters. The structure of the animals is modified in adaptation to this curious habit. The abdomen, which is protected during life by the borrowed sh.e.l.l, is soft and unarmoured, and is spirally twisted. The swimmerets, which have only the function of carrying the eggs in the female, are much reduced, and are usually present only on one side of the body. The uropods no longer form a tail-fan, but are adapted for firmly wedging the hind part of the body into the coils of the sh.e.l.l.

One of the chelipeds is much larger than the other, and serves to block up the opening when the animal withdraws into its shelter. In tropical countries certain Hermit Crabs (_Coen.o.bitidae_) have become adapted to a life on land, and one of these, the well-known Coconut Crab, or Robber Crab (_Birgus latro_), which is the largest species of the tribe, has given up the habit of protecting itself with a sh.e.l.l, and its abdomen has again acquired a strong armour on the upper side. The marine _Lithodidae_--to which the British Stone Crab, _Lithodes maia_ (Plate VIII.) belongs--seem at first sight to have little resemblance to the Hermit Crabs, for they have the abdomen very small, and tucked up under the body as in the true Crabs. Like the Porcellanidae, mentioned above, however, the Lithodidae have only three pairs of walking legs behind the chelipeds, the last pair being feeble and usually folded out of sight within the gill chambers. The relations.h.i.+p of the Lithodidae to the Hermit Crabs is shown by the abdomen, which is more or less twisted to one side, and has swimmerets only on one side in the female, and quite wanting in the male.

[Ill.u.s.tration: _PLATE VIII_

THE "NORTHERN STONE-CRAB," _Lithodes maia_, MUCH REDUCED. THE LAST PAIR OF LEGS ARE FOLDED OUT OF SIGHT IN THE GILL CHAMBERS

(_From Brit. Mus. Guide_)]

The _Hippidea_ are curious little Crabs found burrowing in sandy beaches in the warmer seas. They have the abdomen tucked under the body, and the legs flattened for shovelling the sand.

The BRACHYURA, or true Crabs, form the fourth section of the Reptantia, and are distinguished by having the abdomen reduced to a tail-flap, which is doubled up under the cephalothorax, and is usually without any trace of the uropods which are present in all the groups already mentioned, with the single exception of the Lithodidae. At the sides of the head the side-plates of the carapace become firmly soldered to the "epistome," a plate which lies in front of the mouth, and in this way there is formed the "mouth-frame," within which lie the jaws, covered in by a pair of "folding-doors" formed by the flattened third maxillipeds.

[Ill.u.s.tration: _PLATE IX_

THE COMMON Sh.o.r.e-CRAB (_Carcinus maenas_). (REDUCED)

_Dromia vulgaris_, CARRYING ON ITS BACK A Ma.s.s OF THE SPONGE _Clione celata_. BRITISH. (REDUCED)]

The first tribe of the Brachyura, the _Dromiacea_, comprises a number of Crabs that in many points of structure resemble the Lobsters, and are regarded as the most primitive members of the section. _Dromia vulgaris_ (Plate IX.), a furry, clumsy-looking Crab, occasionally found on our southern coasts, has the last two pairs of legs short and carried up over the back, where they are used for holding a ma.s.s of living sponge which the Crab uses as a cloak to protect and conceal itself. At the sides of the abdomen, wedged in between the telson and the last somite, a pair of small plates may be seen, which are the last vestiges of the uropods. These are wanting in the other tribes of the Brachyura.

[Ill.u.s.tration: _PLATE X_

_Calappa flammea_, BRAZIL. (REDUCED)]

The _Oxystomata_ (Plate X.), which form the second tribe of the Brachyura, are distinguished by the form of the mouth-frame, which is narrowed in front so as to be triangular instead of square in outline.

The pa.s.sages through which the water pa.s.ses out from the gills, which in other Crabs open at the front corners of the mouth-frame, are carried forwards to the front of the head. The Oxystomata are most abundant in tropical seas, but are represented on the British coasts by species of _Ebalia_, small and compact Crabs which are not unlike pebbles of the gravel among which they live.

The remaining Crabs form the tribe _Brachygnatha_, in which the mouth-frame and the maxillipeds that close it are more or less quadrilateral in shape. The tribe is divided into two subtribes, which may be recognized by the general shape of the carapace. In the _Brachyrhyncha_ this is generally rounded or square-cut in front, without a projecting rostrum. In this subtribe are included the great majority of Crabs. The Edible Crab and the Sh.o.r.e Crab (Plate IX.) are familiar examples. In the _Oxyrhyncha_, on the other hand, the carapace is generally narrowed in front, with a projecting rostrum, either simple or forked, and is often armed with spines. In this subtribe are included the long-legged Spider Crabs, several species of which are common on our coasts. The Giant Spider Crab of j.a.pan (Plate XI.) is the largest of living Crustacea.

[Ill.u.s.tration: _PLATE XI_

THE GIANT j.a.pANESE CRAB, _Macrocheira kaempferi_, MALE. THE SCALE OF THE FIGURE IS GIVEN BY A TWO-FOOT RULE PLACED BELOW THE SPECIMEN

(_From Brit. Mus. Guide_)]

The last division of the Eumalacostraca, the HOPLOCARIDA (Plate XII.), is one of very small extent, comprising only a single order (_Stomatopoda_) of very remarkable Crustacea which are common in tropical seas, and of which at least one species, _Squilla desmarestii_, is occasionally captured on the south coast of England. The Stomatopoda are prawn-like Crustaceans, usually with a flattened body, and are easily recognized by the form of the large claws (the second pair of thoracic limbs), in which the last segment shuts down, like the blade of a pocket-knife, on the preceding segment, and forms a very efficient weapon, so that the larger species are not to be handled without caution. The resemblance of these claws to those of the mantis-insect of Southern Europe led to a common Mediterranean species receiving long ago the name _Squilla mantis_ (Plate XII.).

[Ill.u.s.tration: _PLATE XII_

_Squilla mantis_, FROM THE MEDITERRANEAN. ABOUT ONE-HALF NATURAL SIZE

(_From Brit. Mus. Guide_)]

The Stomatopoda have a small carapace, which does not cover the last four thoracic somites, and has in front a small flattened rostrum, attached by a movable hinge, like that of the Leptostraca. The eyes are stalked, and, like the antennules, are attached to a separate movable segment of the front part of the head--a peculiarity not found in any other Crustacea. There are small plate-like gills attached to the bases of some of the thoracic limbs, but the chief organs of respiration are large feathery gills attached to the pleopods or swimmerets.

The Stomatopoda are all found in the sea, generally in shallow water, burrowing in sand or hiding in crevices of rocks or corals. Some species are more than a foot in length.

CHAPTER IV

THE METAMORPHOSES OF CRUSTACEA

The great majority of Crustacea are hatched from the egg in a form very different from that which they finally a.s.sume, and reach the adult state only after pa.s.sing through a series of transformations quite as remarkable as those which a caterpillar undergoes in becoming a b.u.t.terfly, or a tadpole in becoming a frog. Many of these young stages were known for a long time before their larval nature was suspected, and it is one of the curiosities of the history of zoology that, even after the actual changes from one form to another had been observed and described in several Crustacea, many eminent naturalists refused to believe in the possibility of their occurrence. This scepticism was largely due to the fact that the common fresh-water Crayfish, when hatched from the egg, has practically the same structure as the adult, and it was a.s.sumed that other Crustacea were developed in a similar fas.h.i.+on. Although certain cases of metamorphosis had been actually seen and described by naturalists in the eighteenth century, these observations were forgotten or misunderstood till they were confirmed by Mr. J. Vaughan Thompson, a naval surgeon stationed at Cork, the first part of whose "Zoological Researches" was published in 1828. Thompson's statements were much disputed at the time, but they have been confirmed by subsequent research, and it is now known that the majority of Crustacea undergo a more or less extensive metamorphosis after leaving the egg, although, as will be seen later, there are many important exceptions to this rule.

If a fine muslin net be towed at the surface of the sea on a calm day, and the contents turned out into a jar of sea-water, it will usually be found to have captured, among other things, clouds of animated specks, which dance in the water or dart hither and thither with great rapidity.

Many of these specks, when examined with the microscope, will be found to be Crustacea. Besides adult animals belonging to various groups, such as the Copepoda, which pa.s.s the whole of their life swimming near the surface of the sea, there will be numerous larval stages of species which in their adult form live on the sea-bottom. The identification of the species to which the various larvae belong is a matter of considerable difficulty, and, although the general course of development is now well known for all the chief groups of Crustacea, there are very many even of the common British species in which the larval transformations have not yet been worked out in detail.

[Ill.u.s.tration: FIG. 25--LARVAL STAGES OF THE COMMON Sh.o.r.e CRAB (_Carcinus maenas_--SEE PLATE IX.). (Partly after Williamson.)

A, Young zoea, shortly after hatching; B, megalopa stage; C, young Crab.

A 20, B and C 10]

As an example of the larval history of the higher Crustacea, we may take the case of the Common Sh.o.r.e Crab, _Carcinus maenas_ (Fig. 25). The young stages are common in tow-net gatherings round the British coasts in the summer-time. The youngest larvae (Fig. 25, A) are translucent little creatures about one-twentieth of an inch long. They have the head and front part of the body covered by a helmet-shaped carapace, with a long spine standing out from the middle of the back, and another projecting, like a beak, in front.

The narrow abdomen or tail is very flexible, and can be doubled up under the body or stretched out behind; it ends in a forked telson. There are two pairs of swimming limbs, each with endopodite and exopodite, and the short antennules and antennae are seen on either side of the rostrum.

There are a pair of very large compound eyes, which are not set on movable stalks, but are under the front part of the carapace. The two-branched swimming feet are really the first and second maxillipeds (the mandibles, maxillulae, and maxillae, can be found in front of them), but none of the other thoracic limbs are yet developed, and, although the somites of the abdomen are distinct, there are no swimmerets. This type of larva is known as a _zoea_, a name which was given to it when it was supposed to be an independent species of Crustacean. As a matter of fact, the zoea just described is not quite the earliest stage of the Sh.o.r.e Crab, for when hatched from the egg it is without the spines on the carapace, and is slightly different in other respects. A few hours after hatching, however, it casts its skin for the first time, and becomes a fully-formed zoea. It swims rapidly about at the surface of the sea, feeding on the minute floating animals and plants which are found there, and growing in size with repeated castings of its skin. In the later stages of the zoea the rudiments of the hinder thoracic limbs and of the swimmerets appear as little buds. In the next stage (Fig. 25, B) all the appendages are present, the dorsal spine of the carapace has disappeared, the eyes are stalked and movable, and the animal has all the appearance of a little Crab, except that the abdomen is stretched out instead of being tucked up under the body, and the swimmerets are used as paddles in swimming. In this stage the larva, which is known as a _megalopa_, swims at the surface of the sea, but later it sinks to the bottom, and, moulting again, appears as a little Crab (Fig. 25, C), with tucked-up abdomen and swimmerets no longer adapted for locomotion.

[Ill.u.s.tration: FIG. 26--LAST LARVAL STAGE OF THE COMMON PORCELAIN CRAB (_Porcellana longicornis_--SEE FIG. 41, p. 113). 9. (After Sars.)]

Most of the true Crabs (Brachyura) have a larval history similar to that just described, and pa.s.s through zoea and megalopa stages which differ only in details from those of _Carcinus_. The Anomura are also hatched as zoeae, and one of the most remarkable forms common in tow-nettings in British waters is the zoea of the little Porcelain Crabs (_Porcellana_--Fig. 26). In this larva the carapace has two long spines behind, and a rostral spine which is several times as long as the body of the animal. A great development of spines also characterizes the larva of _Munida_ (Fig. 27).

[Ill.u.s.tration: FIG. 27--FIRST LARVAL STAGE OF _Munida rugosa_ (SEE PLATE VI.). 10. (After Sars.)]

The larval form of the Common Lobster has already been described, and it will be noticed that the differences from the adult are much less than in the case of the Crab. From the fact that this larva has swimming exopodites on its legs, like the adult Mysidacea and Euphausiacea (formerly grouped together as "Schizopoda"), it is said to be in the "schizopod stage." The larva of the Norway Lobster (_Nephrops norvegicus_) is essentially of the same type, but the great development of the spines on the abdomen and of the forked telson gives it a striking appearance.

A very remarkable type of larva is found among the Spiny Lobsters and their allies (Scyllaridea). This larva, known by the name of _phyllosoma_ (Fig. 28), is very broad, thin, and leaf-like, and quite transparent, so that some of the larger kinds were formerly known as "Gla.s.s Crabs." The thin oval carapace does not cover the whole of the thoracic region, which is disc-shaped, with four pairs of long slender legs, each with an exopodite. The abdomen is relatively small. The intermediate stages between the phyllosoma and the adult are still very imperfectly known. In tropical seas phyllosoma larvae of large size are found, sometimes reaching two or three inches in length. The larva of the Common Spiny Lobster (_Palinurus vulgaris_), however, does not exceed half an inch in length.

[Ill.u.s.tration: FIG. 28--THE PHYLLOSOMA LARVA OF THE COMMON SPINY LOBSTER (_Palinurus vulgaris_--SEE PLATE V.). MUCH ENLARGED. (After J. T.