The Life of Crustacea Part 5

Cunningham.)]

The Shrimps and Prawns of the tribe Caridea are mostly hatched as zoeae, and pa.s.s through a "schizopod" stage comparable to that of the Lobster, in which they swim by means of exopodites on the legs. Some of the Prawns belonging to the tribe Penaeidea, however, have a still more remarkable metamorphosis, which is very important on account of the resemblance of the earlier stages to those of the lower Crustacea. Fritz Muller discovered in 1863 that _Penaeus_ is hatched from the egg as a _Nauplius_ (Fig. 29, A), a form of larva which was previously known among the Copepoda, Branchiopoda, and Cirripedes. The nauplius, unlike the larvae which we have been considering, has an unsegmented body, and has only three pairs of limbs. The body is pear-shaped in outline, and near the front end is seen the median eye, sometimes called, from its presence in this type of larva, the "nauplius-eye"; the paired eyes are not yet developed. The three pairs of limbs are shown by their later development to be the antennules, antennae, and mandibles; the first pair are unbranched, the second and third divided into exopodite and endopodite. It is interesting to notice that the antennae and mandibles, which in the adult animal are so widely different that it is difficult to trace any resemblance between them, are in the nauplius almost identical in form. Further, the antennae, instead of being placed in front of the mouth as in the adult, lie on either side of it, and each has at its base a hooked spine which projects inwards and serves for seizing particles of food and pa.s.sing them into the mouth; the antennae of the nauplius, in fact, serve as jaws, while it is only later that the mandibles take on this function.

[Ill.u.s.tration: FIG. 29--LARVAL STAGES OF THE PRAWN--_Penaeus_ (SEE PLATE IV.). 45. (After F. Muller.)

A, Nauplius; B, young zoea; C, older zoea; D, early "schizopod" stage]

In the further development of the larva, the body increases in length and becomes divided into somites which increase in number by new somites appearing behind those already marked off; the rudiments of the limbs also appear in regular order from before backwards; the dorsal s.h.i.+eld of the nauplius grows out into a carapace, beneath which the paired eyes begin to develop in front. Thus after pa.s.sing through _metanauplius_ and _protozoea_ stages (Fig. 29, B) the larva becomes a _zoea_ (Fig. 29, C), resembling that of the Crab already described in that the swimming organs are the maxillipeds, but differing in having the uropods well developed and forming a tail-fan at the end of the abdomen, the hinder thoracic somites marked off and their appendages present as rudiments, and the stalked eyes free from the carapace. This is followed by a _schizopod_ stage (Fig. 29, D), in which the prawn-like shape is a.s.sumed and the thoracic legs have large exopodites used for swimming. Later these exopodites diminish in size, though they do not quite disappear in the adult _Penaeus_, and the function of swimming organs is taken over by the abdominal swimmerets.

In _Penaeus_ the larvae are of comparatively simple form, but in the allied genus _Sergestes_ the zoea has a very remarkable appearance. The carapace is armed with long spines, each bearing two comb-like rows of secondary spines. The development of spines and other outgrowths of the surface of the body is a very common characteristic of organisms that, like these larvae, float or swim in the open sea; its probable significance will be discussed in a later chapter.

The shrimp-like Euphausiacea have a larval development very like that of _Penaeus_. Most, if not all, of the species are hatched from the egg in the nauplius stage, and pa.s.s through stages very similar to those described above. The adult animals, however, may be said to remain in the "schizopod" stage, since the exopodites of the thoracic legs remain large and are used in swimming.

[Ill.u.s.tration: FIG. 30--NEWLY-HATCHED YOUNG OF A CRAYFISH (_Astacus fluviatilis_). ENLARGED]

Even among the Decapoda, however, there are many species that are hatched from the egg in a form that does not differ essentially from the adult, and are therefore said to have a direct development. This is often the case with species which live in fresh water or in the depths of the sea. For example, the young of the fresh-water Crayfish (Fig.

30), when hatched, possess all the appendages of the adult except the first pair of swimmerets and the uropods, or outer plates of the tail-fan. The carapace is almost globular, owing to the presence inside the body of a large amount of food-yolk, which supplies the nourishment necessary for the young animal in the early stages of its development.

The chelae have hooked tips, by means of which the young animal clings securely to the swimmerets of the mother. After a time it moults, and the uropods are set free, the chelae lose their hooked tips, the carapace a.s.sumes nearly its final shape (the food-yolk having been largely absorbed), and the young Crayfish leaves the protection of its parent, to s.h.i.+ft for itself. The essential point of difference between the development of the Crayfish and that of the closely related Lobster (see Fig. 8, p. 28) is not so much that the changes in structure which occur after hatching are less profound in the former case, but that there is no free larval stage. In the Lobster the earlier stages are capable of independent existence, and they differ from the full-grown animal not only in structure, but also in habits, swimming at the surface instead of creeping at the bottom of the sea.

A similar case to that of the Crayfishes is found in the River Crabs of tropical countries, belonging to the family Potamonidae. These Crabs are as closely related to some marine Crabs as are the Crayfishes to the Lobsters, yet the difference in their mode of development is even more p.r.o.nounced. Instead of beginning life as minute pelagic zoeae, they leave the shelter of the mother's abdomen as perfectly-formed little Crabs (Fig. 31).

[Ill.u.s.tration: FIG. 31--YOUNG SPECIMEN OF AN AFRICAN RIVER CRAB (_Potamon johnstoni_), TAKEN FROM THE ABDOMEN OF THE MOTHER. MUCH ENLARGED

The adult of an allied species is figured on Plate XXIII]

Amongst the Decapoda, instances of direct development like those just described are exceptional, but in some of the other orders of the Malacostraca direct development is the rule. In the great division Peracarida, as we have already seen, the females are provided with a pouch, or marsupium (from which the name of the division is derived), in which the eggs are carried. Within this pouch the young undergo the whole of their development, and they only leave it, as a rule, when they have attained the structure of the adults. Among the more familiar representatives of this division, the Sand-hoppers (Amphipoda), the Woodlice (Isopoda), and the Opossum Shrimps (Mysidacea), may be mentioned as examples of this mode of development. The Woodlice and their immediate allies differ a little from the other members of the division in the fact that the young leave the brood-pouch with the last pair of legs still undeveloped, though in other respects they are like miniature adults.

In those Crustacea which have a direct development without free-swimming larval stages, it is sometimes possible to find traces of such stages in the early development of the embryo. This is shown most clearly, perhaps, in the Opossum Shrimps (Mysidacea). In these the embryo becomes free from the egg-membrane (or may, in a sense, be said to "hatch") at a very early stage, and lies free within the brood-pouch as a maggot-shaped body, on which three pairs of rudimentary limbs can be made out. The later development shows that these three rudiments correspond to the antennules, antennae, and mandibles, so that the maggot-shaped embryo is, in fact, a disguised nauplius without the power of swimming or of leading an independent existence. In other cases--as, for instance, in the Crayfish, where the earlier stages are confined within the egg-membrane (or "egg-sh.e.l.l")--the nauplius stage, although more difficult to examine, is quite as well marked.

Of the other groups of the Malacostraca, the Syncarida and Leptostraca are hatched in nearly the adult form, but the Stomatopoda have a long series of larval stages. These larvae (Fig. 32) are all distinguished by the large size of the carapace, which in some cases envelops the greater part of the body. Some Stomatopod larvae, in the warmer seas, attain to a relatively great size, sometimes exceeding 2 inches in length, and their gla.s.s-like transparency gives them a very striking appearance.

[Ill.u.s.tration: FIG. 32--EARLY LARVAL STAGE OF A SPECIES OF SQUILLA, PROBABLY _S. dubia_. 10. (After Brooks.)]

As we have seen, it is exceptional to find a free-swimming nauplius larva among the Malacostraca, but it is the commonest larval stage in the other subcla.s.ses of Crustacea. Most of the Branchiopoda are hatched in this form (Fig. 33), and reach the adult state by a very gradual series of changes in which new somites and appendages are added in regular order from before backwards till the full number is reached. The Water-fleas (Cladocera), however, differ from most of the other Branchiopoda in having a direct development. The eggs are carried in a brood-pouch under the back of the carapace, and in this the embryos undergo their development. In the common _Daphnia_, for instance, numerous eggs or young can generally be seen through the transparent carapace (see Fig. 12, p. 37).

[Ill.u.s.tration: FIG. 33--LARVAL STAGES OF THE BRINE SHRIMP (_Artemia salina_). (After Sars.)

A, Nauplius, just hatched; B-E, later stages, showing progressive increase in number of somites and appendages. The adult form of this species is shown in Fig. 55, p. 164]

Many of the Ostracoda have a direct development, but in some cases the young animal, on hatching, has only the first three pairs of appendages, and is therefore regarded as a nauplius, although it possesses a bivalved sh.e.l.l like that of the adult, and is very unlike the nauplius larvae of other Crustacea.

[Ill.u.s.tration: FIG. 34--EARLY NAUPLIUS LARVA OF A COPEPOD (_Cyclops_).

MUCH ENLARGED. (From Lankester's "Treatise on Zoology.")

_a'_, Antennule; _a''_, antenna; _gn_, jaw-spine of antenna; _lbr_, upper lip; _md_, mandible]

Most of the Copepoda also leave the egg in the nauplius stage; and, indeed, it was to the young of the common fresh-water _Cyclops_ (Fig.

34) that the name of _Nauplius_ was first given by the Danish naturalist, O. F. Muller, in the eighteenth century, in the belief that it was an adult and independent species of Crustacea. In the Copepoda, the changes which transform the nauplius into the adult are gradual, and consist chiefly in the successive addition of new somites and appendages.

[Ill.u.s.tration: FIG. 35--LARVAL STAGES OF THE COMMON ROCK BARNACLE (_Bala.n.u.s balanoides_--SEE PLATE III.)

A, Nauplius stage (after Hoek); B, cypris stage (after Spence Bate)]

The development of the Cirripedia is of special interest, since it was the discovery of the larval stages by J. Vaughan Thompson that first demonstrated to naturalists that the Barnacles were Crustacea and not, as had been supposed, Molluscs. The earliest stage is generally a nauplius (Fig. 35, A) of very peculiar and characteristic form, with a pair of horns projecting sideways from the front corners of the dorsal s.h.i.+eld, and a forked spine on the under-side behind. The later development is very unlike those which have been described above, for after a series of nauplius stages the larva pa.s.ses suddenly, at a single moult, into a stage in which the body and limbs are enclosed in a bivalved sh.e.l.l (Fig. 35, B). From the superficial resemblance of the sh.e.l.l to that of an Ostracod, this is known as the _cypris_ stage.

Through the valves of the sh.e.l.l a pair of large compound eyes can be seen, as well as six pairs of two-branched swimming feet, while in front a pair of antennules project between the valves. On each antennule is a sucker-like disc by means of which the larva, after swimming freely for some time, attaches itself to a stone or some other object, where it remains fixed for the rest of its life. A cementing substance produced by a gland at the base of the antennules attaches the front part of the head firmly to the support; the valves of the sh.e.l.l are cast off, and replaced by the rudimentary valves of the adult sh.e.l.l; the six pairs of swimming feet grow out into tendril-like cirri; the compound eyes disappear, and the animal a.s.sumes the structure of the adult.

The parasitic Rhizocephala have a very remarkable life-history, which will be described in a later chapter; but it may be mentioned here that their free-swimming larval stages resemble very closely those of the ordinary Barnacles. It was the discovery of this fact which led to its being recognized that the Rhizocephala are highly modified and degenerate Cirripedes, although their structure in the adult state gives little evidence of their affinities.

A number of interesting problems in speculative biology are suggested by the larval stages of Crustacea. A full discussion of these problems would involve matters too technical for these pages, but some indication of the broader issues may be attempted.

The obvious question, Why do some Crustacea pa.s.s through a complicated metamorphosis while others do not? is, like many obvious and simple questions, one of the most difficult to answer. It will be pointed out later, in dealing with the fresh-water Crustacea, that one of the most general characters of fresh-water animals as compared with their marine allies is the absence of free-swimming larval stages. This applies, for instance, to the case of the Crayfishes and the marine Lobsters, and to that of the River Crabs, as compared with those which live in the sea.

But it does not apply to all fresh-water Crustacea, and, on the other hand, there are many cases of direct development in marine species.

Some of the advantages gained by the possession of free-swimming larval stages are obvious enough. Many Crustacea which live on the sea-bottom, and are not very powerful swimmers, have their progeny scattered far and wide by winds and currents while in the surface-living larval stages. In the extreme case of the Barnacles, which are fixed to one spot when adult, a locomotive larval stage is clearly a necessity. But, here as elsewhere, to demonstrate the usefulness of any character is to go only a very little way towards explaining its origin. Moreover, the mere necessity for a locomotive larva throws no light on the remarkable resemblances between the larval stages of widely different species. In the adult state, a Branchiopod, a Copepod, an Ostracod, a Barnacle, and a Penaeid Prawn, are separated by enormous differences of form and structure; yet, as we have seen, all these are hatched from the egg as six-limbed nauplius larvae differing from each other only in trivial details. It seems hardly possible to imagine any other interpretation of this very striking fact than is afforded by the theory of Evolution. We are forced to a.s.sume that all these diverse forms of Crustacea are descended from very similar or identical ancestral types, and that the modifications arising in the course of their evolution have affected the adult but not the larval stages. Some naturalists would go farther than this, and would apply the so-called "theory of recapitulation" to the larval stages of the Crustacea. According to this theory, the stages in the development of any animal tend to recapitulate, more or less closely, the history of the race. Thus it is a.s.sumed, for instance, that the nauplius reproduces the structure of a six-limbed ancestral form, from which, in the distant past, all the diverse branches of the Crustacean cla.s.s took their origin. There are, however, considerable difficulties in the way of this view. That some such ancestral type did exist may be regarded as tolerably certain; that it resembled in its adult state the nauplius larvae of present-day Crustacea is, on the whole, unlikely; but it is not at all improbable, whatever its adult structure may have been, that it hatched from the egg as a nauplius larva.

With regard to some of the other larval forms, it is possible to speak with a little more confidence. There are good grounds for believing, apart from the evidence of development, that the Lobster and its allies have descended from Crustacea which, like the existing Euphausiacea, possessed swimming branches (exopodites) on the thoracic legs; and there seems no reason to doubt that the "schizopod" larva of the Lobster does recapitulate this stage in the evolution of the race. On the other hand, it is impossible to believe that any of the ancestors of the Sh.o.r.e Crab resembled, even remotely, the zoea stage with which the life-history of the individual now begins.

CHAPTER V

CRUSTACEA OF THE SEASh.o.r.e

The tract of seash.o.r.e which is laid bare by the retreat of the tide offers on most coasts a rich collecting-ground to the student of Crustacea. In places where shelving, weed-covered rocks run out to sea, innumerable Crustacea have their home in the rock-pools, or lurk in crannies awaiting the return of the tide. On sandy beaches, at first sight apparently barren of life, a closer search will reveal a whole fauna, amongst which burrowing Crustacea of various orders are prominent. Further, the sh.o.r.e collector will find from time to time stray specimens of forms that have their proper habitat beyond low-tide mark, and occasionally their remains are thrown in quant.i.ties on the beach by storms. It is convenient, therefore, to treat the Crustacea of the sh.o.r.e as a sample of those inhabiting the shallower waters of the ocean. In these shallower waters--down to the limit where light no longer penetrates from above, where vegetable life ceases, and where the strangely modified inhabitants of the deep sea begin to appear--the sea-bottom is perhaps the most densely populated of all parts of the earth's surface. Nowhere, at all events, do we find so wide a range of animal forms, from the simplest organisms (Protozoa) up to highly-organized Vertebrates. Nowhere, perhaps, is the struggle for existence more keen, and it is not without justice that some naturalists have regarded the shallow waters of the sea as "one of the great battle-fields of life," where, in the long course of evolution, the main branches of the animal kingdom have had their origin.

Conspicuous among the animals of this region are Crustacea of all sorts and sizes. To identify all the species that may be obtained in a single haul of the dredge in British seas would sometimes be a hard task even for the most expert student of the group. Our present purpose, however, is not to compile a faunistic catalogue, but merely to give some idea of the endless diversity of form, and to note a few of the "s.h.i.+fts for a living"--of the ways in which structure and habit are adapted to the conditions of life in the Crustacea of the sh.o.r.e and of shallow water.

Though it might seem that the heavily armoured Lobsters and the larger Crabs would be sufficiently protected against most enemies when once they have attained their full size, yet they are preyed upon by the Octopus, which seizes them with its suckers and pierces their armour with its powerful beak, injecting a poison that paralyzes its victims.

Some years ago a "plague" of Octopus very seriously affected the Lobster fishery in the English Channel. To escape from enemies such as these, the Lobsters and many Crabs have the habit of lurking in crevices of the rocks, while in case of sudden alarm the Lobster may escape from danger by swimming, or rather darting, with great swiftness, tail foremost, through the water by powerful strokes of the abdomen and tail-fan. In the more lightly armed Prawns and other Crustacea of the tribe Natantia, which are characteristically swimmers, the power of rapid motion is probably the chief means of protection against enemies. There is reason to believe that the Lobsters have been derived from prawn-like swimming forms which have sacrificed some of their agility in developing their heavy armour-plating, retaining, however, the power of sudden and rapid motion in emergency. This power, again, has been lost by the typical Crabs (Brachyura), in which the abdomen is reduced in size and without a tail-fan, so as to be useless for swimming. While most of the Crabs, however, are somewhat slow of movement, trusting to their armour and their powerful pincers for defence, the Swimming Crabs (Portunidae--Plate XIII.) have reacquired the power of swimming by means of the paddle-shaped legs of the last pair. Some of the tropical species of Portunidae are probably the most expert swimmers among the Crustacea, and are described as shooting through the water like fish.

[Ill.u.s.tration: FIG. 36--A COMMON HERMIT CRAB (_Eupagurus bernhardus_) REMOVED FROM THE Sh.e.l.l]

The Lobster's habit of seeking shelter in rock-crevices or under stones is one which is shared by a very large number of sh.o.r.e Crustacea. From some primitive kind of Lobster which discovered the advantages of a portable shelter have been derived the Hermit Crabs. In rock-pools one may often see whelk or periwinkle sh.e.l.ls tumbling about with an activity quite foreign to the nature of their original molluscan inhabitants, and closer examination will show that each contains a Hermit Crab, which retreats into the sh.e.l.l when disturbed. If extracted from the sh.e.l.l, the Crab (Fig. 36) can be seen to be most beautifully adapted to its peculiar mode of life. The abdomen is soft and spirally twisted to fit into the interior of the spiral sh.e.l.l, and the uropods, instead of forming a tail-fan, are modified into holding organs, with roughened, file-like surfaces which can be pressed outwards against the walls of the sh.e.l.l, and wedge the body so firmly that an attempt to drag the animal forcibly from its retreat often results in tearing it in half. The front part of the body, which is exposed when the animal is walking, retains its sh.e.l.ly armour. One of the pincer-claws, most commonly the right, is much larger than the other, and serves to block the opening of the sh.e.l.l when the body is withdrawn into it. The next two pairs of legs are long and slender, and are used for walking; but the last two pairs are short, with a roughened surface at the end, and serve to steady the body in the mouth of the sh.e.l.l. The swimmerets on the right side of the body, which is pressed against the central pillar of the sh.e.l.l, have disappeared, but those of the left side remain.

As the Hermit grows, it is necessary for him to remove from time to time into a larger dwelling. It has been stated that he will sometimes dispossess the rightful owner of a whelk-sh.e.l.l for this purpose, dragging him out piecemeal and eating him; but other observers deny that this ever happens, and in most cases, at all events, the Hermit is content to wait until he finds an empty sh.e.l.l of suitable size. After turning this over and exploring the interior with his claws, to satisfy himself that it is unoccupied, he deftly whips the unprotected hinder part of his body into the new habitation, keeping hold of the old one meanwhile, so that he can return to it if the other proves unsuitable.

The Hermits are very pugnacious, and fight with one another for the possession of desirable sh.e.l.ls, the victor dragging his opponent out and establis.h.i.+ng himself in his place. Besides appropriating the sh.e.l.l of a dead Mollusc, many Hermits seem to go into partners.h.i.+p with living animals of various kinds, and some of these a.s.sociations will be noticed in a later chapter. A number of species adopt other dwellings than molluscan sh.e.l.ls, and some tropical Hermits, for instance, are found living in the cavities of water-logged stems of bamboo (Fig. 37); while others, relinquis.h.i.+ng the advantages of a portable shelter, live in holes in corals or in the ca.n.a.ls of living sponges. Although in some of these cases the body is straight, it usually shows traces of its original adaptation to a spiral sh.e.l.l in having no swimmerets on the left side.

[Ill.u.s.tration: FIG. 37--_Pylocheles miersii_, A SYMMETRICAL HERMIT CRAB.

(After Alc.o.c.k.)

The upper figure gives an end view of the animal lodged in a tube of water-logged mangrove or bamboo, its large claws closing the opening.

The lower figure shows the animal removed from its shelter.]

The only Hermits which have a full series of swimmerets are the primitive Pylochelidae (Fig. 37), which come very near to what we imagine the ancestral form of the group to have been like, and can hardly be separated from the mud-burrowing, lobster-like Thala.s.sinidea. A few Hermits have given up altogether the use of any protective covering. One of these is the Coconut Crab (_Birgus_), to be mentioned when we come to deal with the Crustacea of the land. Another is the Stone Crab (_Lithodes_--Plate VIII.) of our own seas, and its kindred, which have redeveloped sh.e.l.ly plates on the back of the abdomen, but carry it doubled up under the body like the true Crabs. These also preserve some traces of the original twisting of the abdomen, and have swimmerets only on one side.

Some Crustacea construct habitations for themselves. On turning over a flat stone between tide-marks, one often finds a little ma.s.s of bits of weed and rubbish attached to it, and if this be torn open a greenish-brown, shrimp-like animal, about three-quarters of an inch long, is seen slithering away on its side. This is an Amphipod (_Amphithoe rubricata_) which builds the shelter for itself, sticking the fragments together with threads of a cementing material produced by glands on the surface of its body and legs. Other Amphipods construct more neatly finished tubular dwellings of mud, or even of small stones, which are attached to sea-weeds and the like; and some make portable shelters of the same kind, which they carry about with them like the caddis-worms of fresh-water streams.