The Life of Crustacea Part 13

In the warmer seas the large Prawns of the genus _Penaeus_ are of considerable importance. Thus, in the Mediterranean countries, _Penaeus caramote_ (Plate IV.) is highly esteemed for food, and _P. setifer_ and _P. brasiliensis_ are largely consumed in the Southern United States.

_P. monodon_ and other species are eaten in India. An attempt has been made to send a species of the same genus (apparently _P. indicus_) in a frozen state from Queensland to the London market.

Numerous other species of Natantia are used for food in various parts of the world, but the only ones that need be further mentioned here are the River Prawns of the genus _Palaemon_, which are abundant in the fresh waters of most tropical countries, and sometimes grow to a very large size. They are generally distinguished by the fact that the legs of the second pair are very long, forming powerful pincer-claws. In the West Indies and Central America, _P. jamaicensis_ (Plate XXI.), which reaches a length of 10 inches exclusive of the great claws, is sold in the markets, while in India and elsewhere in the East _P. carcinus_, which grows to an even greater size, and other smaller species, are used for food. The fresh-water Prawns of the family Atyidae, on account of their small size, are not of much importance from this point of view, but Professor Hickson states that the little _Caridina nilotica_, a very widely-distributed species, is eaten in Celebes.

[Ill.u.s.tration: _PLATE x.x.xI_

THE COMMON EDIBLE CRAB, _Cancer pagurus_. BRITISH. (MUCH REDUCED)]

Among British Crustacea, the next in importance to the Lobster as an article of food is the Edible Crab, _Cancer pagurus_ (Plate x.x.xI.), known in Scotland as the "Partan." Like the Lobster, it is found on rocky coasts in shallow water, and young specimens are often taken between tide-marks. It grows to a size of more than 10 inches across the sh.e.l.l, and may reach a weight of 12 pounds. The means used for its capture are the same as in the case of the Lobster, and the fishery is of considerable importance on many parts of the British coasts. On the other hand, a Connemara fisherman, who was using these Crabs for bait, received with incredulity the statement that they were good for human food!

The Sh.o.r.e Crab, _Carcinus maenas_ (Plate IX.), is not of much importance as food in this country, although it is recorded that fifty years ago great numbers were brought to the London market. On the sh.o.r.es of the Mediterranean and Adriatic, however, and especially in Venice, this species is regarded as a delicacy, particularly in the soft-sh.e.l.led state after moulting.

On the Atlantic coast of North America, the most important edible Crustacean after the Lobster is the "Blue Crab" (_Callinectes sapidus_), one of the Swimming Crabs (Portunidae). This is consumed in large quant.i.ties, especially in the soft-sh.e.l.led state. Several other species of Crabs are eaten in America, including the little "Oyster Crab," a species of _Pinnotheres_ living in the American Oyster. From its small size, and the difficulty of obtaining it in numbers, it is a very costly delicacy.

In the East Indies the most important edible Crabs are various species of Portunidae, especially the large _Scylla serrata_ and _Neptunus pelagicus_.

Except as food, the Crustacea are of very little direct use to man.

Almost the only instance in which they are otherwise utilized is in the case of a species of sessile Barnacle (_Bala.n.u.s_) which is cultivated in j.a.pan for use as manure. The method of culture has been described by Professor Mitsukuri. Bunches of bamboo "collectors," like those used for the collection of oyster-spat, are fixed into the ground on tidal flats.

After two or three months they are taken up, and the Barnacles with which they have become covered are beaten off and sold for use as manure.

Apart from their direct utility, however, the Crustacea are indirectly of great importance as providing a large part of the food-supply of marketable fishes. From this point of view, a study of the habits and distribution of the commoner species may be of practical value in throwing light on the migrations and other obscure points in the life-history of the fishes that prey upon them. As an example of this, we may refer to some investigations on the Mackerel fishery recently carried out by the naturalists of the Marine Biological a.s.sociation at Plymouth. In the spring and early summer months the Mackerel migrate into insh.o.r.e waters for the purpose of sp.a.w.ning. During this period the fish congregate in shoals at the surface of the sea, and are captured in drift-nets. The extent of this "shoaling" varies greatly from year to year, and determines whether the season shall be a profitable one for the fishermen or not. When shoaling, the fish feed exclusively on plankton, consisting largely of Copepoda, and it has been shown by Mr.

G. E. Bullen that the fluctuations in the yield of the Mackerel fishery from year to year follow very closely the fluctuations in the abundance of the Copepod plankton on the fis.h.i.+ng-grounds. The investigation has been carried a step farther by Dr. E. J. Allen, who points out that the abundance of Copepods is determined by the abundance of the Diatoms and other minute vegetable organisms of the plankton. These organisms are very largely influenced by the amount of suns.h.i.+ne during the period of their development in the earlier months of the year. Dr. Allen gives a diagram showing for each of seven years (1902-1908) the average number of hours of bright suns.h.i.+ne during the months of February and March in the South-West of England. With this he compares the number of fish caught in the month of May in each of these years by certain vessels engaged in the western Mackerel fishery. The correspondence between the two is very striking indeed, and justifies his conclusion that the amount of suns.h.i.+ne in the early months of the year determines the abundance of the vegetable life of the plankton, and through it of the Copepods and other animals which form the bulk of the plankton a little later in the year; and although there are doubtless other influences at work determining the success or failure of the fishery, it is largely a matter of the richness or poverty of the plankton harvest.

None of the Crustacea can be regarded as directly harmful to man. They have not the power of inflicting envenomed wounds which renders some other Arthropods, such as Scorpions, some Spiders, Centipedes, and Insects, formidable in spite of their small size; and although blood-curdling tales of the ferocity of the Land Crabs are to be found in the accounts of old voyages, even the largest of these is hardly an antagonist to be dreaded.

A considerable number of invertebrate animals, not of themselves noxious, are now known to be the indirect cause of much serious injury to human life by harbouring and disseminating organisms which produce disease. The progress of research is adding, almost every day, to the number of species known to be disease-carriers, and it is possible that in the future some Crustacea as yet unsuspected may be added to the list.

At present, however, there is only one case in which a Crustacean has been shown to be concerned in the transmission of a parasite of man. The "Guinea-worm," _Filaria_ (or _Dracunculus_) _medinensis_, is a parasite belonging to the group of "Thread-worms" (Nematoda) which causes dangerous abscesses under the skin of the legs in many parts of tropical Africa. It has been shown that the embryos of the worm, which are discharged in vast numbers on the bursting of the abscess, do not develop unless they fall into water containing certain species of the Copepod _Cyclops_ (see Fig. 14, p. 39). In some way not yet understood, the embryos penetrate into the body cavity of the _Cyclops_, where they undergo a metamorphosis. For their further development it is necessary that the _Cyclops_ should be swallowed by man, as may easily happen in drinking water from a pond. When the _Cyclops_ is digested the larval worms are set free, and they bore their way through the tissues of their human host till they reach the place (generally under the skin of the leg) where they complete their development and produce the innumerable embryos that are set free in the way just described.

A few Crustacea inflict a certain amount of injury on man in more indirect ways. In tropical countries, Land Crabs are often troublesome in gardens, and may cause serious damage to young plants in sugar-cane plantations and rice-fields. In gardens in this country, the Woodlice, as already mentioned, are sometimes destructive to seedlings and delicate plants. The little fresh-water Isopod, _Asellus aquaticus_, is accused of destroying the nets used in fis.h.i.+ng for Pollan in Lough Neagh in Ireland.

[Ill.u.s.tration: FIG. 80--THE GRIBBLE (_Limnoria lignorum_). MUCH ENLARGED (From British Museum Guide, after Sars.)]

Probably the most important of all Crustacea, however, as regards their destructive activity, are the species which bore into wood, and sometimes do extensive damage to the submerged timber of piers, jetties, and similar structures. On our own coast the most destructive is a little Isopod known as the "Gribble" (_Limnoria lignorum_--Fig. 80), which is distributed from Norway to the Black Sea, and occurs also on the Atlantic coast of North America. Several species of the same genus having similar habits are found in other parts of the world. The Gribble was first discovered as a British species by Robert Stevenson, the celebrated lighthouse engineer, who found it in 1811 destroying the woodwork employed in the erection of the Bell Rock Lighthouse, and sent specimens to Dr. Leach of the British Museum. The animal is only about one-eighth of an inch in length, and its cylindrical burrow is about one-fifteenth of an inch in diameter, and penetrates for a depth of one or two inches. The excavation of the wood is effected by means of the mandibles, which are unusually strong; and when the animals are numerous the burrows are driven so close together that the surface of the wood is reduced to a spongy ma.s.s which is rapidly washed away by the waves (Plate x.x.xII.). The Gribble is often accompanied by another Crustacean of similar habits, the Amphipod _Chelura terebrans_. The latter is about one-fifth of an inch in length, and differs from most Amphipods in having the body somewhat flattened from above downwards instead of from side to side. The burrows made by _Chelura_ are shallower than those of the Gribble, and generally run more or less parallel to the surface of the wood.

[Ill.u.s.tration: _PLATE x.x.xII_

PIECE OF TIMBER FROM RYDE PIER SHOWING DAMAGE CAUSED BY _Limnoria_ AND _Chelura_

(_From Brit. Mus. Guide_)]

CHAPTER XII

CRUSTACEA OF THE PAST

Since the acceptance by naturalists of the theory of Evolution as indicating the mode of origin of the various forms of life now existing, one of the chief lines of biological investigation has had for its object the reconstruction of the pedigree (or, as it is called, the "phylogeny") of the larger groups of the animal and vegetable kingdoms.

In attempting to do this, there are three main sources from which evidence may be drawn. The results of Comparative Anatomy enable us to decide with more or less confidence as to the degrees of relations.h.i.+p between the groups of organisms, and to distinguish between the more primitive and the more specialized; the study of Embryology is, at least, an indispensable adjunct to Comparative Anatomy, even if it does not, as was once supposed, give us an actual recapitulation of ancestral history; and, finally, the study of Fossil Remains holds out the hope that we may be able to find the ancestral types themselves.

It is clear that evidence from the last-named source, when it is available, is the most important of all, since the order of succession of the various types is given by that of the rock strata in which they occur, and we can be quite certain that we are dealing, if not with the actual ancestors, at least with the forerunners of existing species. The "imperfection of the geological record," however, is so great that the organisms preserved in the fossil state represent only an insignificant part of the whole number of organisms that have lived on the globe since life began; and it is not surprising, therefore, that in many groups the study of fossils has. .h.i.therto afforded little help towards the working out of their genealogical history. Thus, among Crustacea there are many important groups such as the Copepoda, which are entirely unknown as fossils, their small and delicate bodies being ill adapted for preservation, although there is every reason to suppose that they are a very primitive and very ancient group. In many fossil Crustacea only the hard sh.e.l.l or carapace has been preserved, the appendages being lost or represented only by indecipherable fragments, and in some cases it is hardly possible to guess at the affinities of the animals. Further, several important groups are already represented in some of the oldest of the fossil-bearing rocks at present known, and the differentiation of these groups must have taken place in the dark ages before the record of the fossils begins. In spite of these disadvantages, however, the study of fossil Crustacea does throw considerable light on the evolution of the group, and it is likely that interesting results in this direction await future investigations.

[Ill.u.s.tration: FIG. 81--RESTORATION OF A TRILOBITE (_Triarthrus becki_), SHOWING THE APPENDAGES. UPPER SIDE ON RIGHT, UNDER-SIDE ON LEFT.

SLIGHTLY ENLARGED. (After Beecher.)]

In the earliest fossiliferous rocks the most abundant and important Arthropods are the Trilobites (Fig. 81), an extinct group which appears to have been related to the primitive Crustacea. The name Trilobite refers to the three-lobed form of the body when seen from the dorsal side, most species having a pair of grooves running lengthwise which divide off a middle lobe containing the princ.i.p.al organs of the body from two lateral "pleural" expansions covering the limbs. The head-s.h.i.+eld shows indications of being composed of five segments, and bears a pair of sessile compound eyes. It is followed by a number (up to twenty-six) of free somites, and the body ends in a tail-s.h.i.+eld, or "pygidium," which is often plainly composed of several somites fused together. Although Trilobites are among the commonest and most familiar of fossils in the older rocks, the nature of their appendages remained quite unknown until within recent years, when specimens of several species showing the structure of the limbs and under-side of the body were discovered in America. From these it appears that the head bore in front a pair of long thread-like antennae and four pairs of two-branched appendages, each with a jaw process, or "gnathobase," turned towards the mouth, which is covered below by a large anterior lip, or "hypostome."

It seems probable that the five pairs of head-appendages correspond respectively to the antennules, antennae, mandibles, maxillulae, and maxillae, of Crustacea; but the second pair appear to have acted as jaws, retaining the gnathobase which, among Crustacea, is only hinted at by the hooked spine on the antenna of the nauplius larva.

Each of the free somites and of those forming the tail-s.h.i.+eld bears a pair of two-branched appendages, not differing greatly from the posterior appendages of the head, but becoming smaller and more flattened towards the hinder end of the body. The numerous genera and species of Trilobites present great differences in the form and ornamentation of the dorsal surface of the body, and it is probable that considerable differences may also have existed in the structure of the limbs, which are only known in two or three species. Some Trilobites are among the most ancient of known fossils, being found in rocks of the Lower Cambrian epoch. The group reaches its maximum development in the Ordovician, and the number of the species and size of the individuals gradually diminish through the Silurian and Devonian till they become extinct at the close of the Carboniferous epoch, except for a single species found in rocks of Permian age in America.

Although zoologists are not all agreed as to the precise systematic place to be a.s.signed to the Trilobites, there can be little doubt that they were related more or less closely to the most primitive Crustacea, and they are of special interest as preserving for us the stage in which the second pair of appendages were still used as biting jaws, and had not moved forwards in front of the mouth to become antennae, as in all living Crustacea.

Contemporary with some of the earliest Trilobites, however, are undoubted Crustacea, which, so far as we know their structure, are not very different from types now living. In the Cambrian epoch the Branchiopoda appear to be represented by _Protocaris_, which in its general form resembles _Apus_; and there are a variety of genera and species of Ostracoda, although, since their sh.e.l.ls alone are preserved, it is not possible to determine their exact relations to existing forms.

In the succeeding Ordovician and Silurian epochs we first meet with the remains of Barnacles, and it is interesting to note that some of them are referred to the genera _Pollicipes_ and _Scalpellum_, which are represented by numerous species in the seas of the present day. Along with these, however, are some strange-looking forms (_Turrilepas_, etc.), having the body covered with rows of overlapping plates. If these are really Cirripedes, they must have differed considerably in structure from the more modern types.

The Malacostraca are more interesting from the point of view of palaeontology than the other subcla.s.ses of Crustacea, since the evolution of the group appears to have taken place within the period covered by the fossil records, and it is possible to trace the course of that evolution--at least, in its broad outlines. It has already been pointed out that the most primitive of existing Malacostraca are the Phyllocarida (_Nebalia_ and its allies), which are in several respects intermediate between the higher Malacostraca and the Branchiopoda; and it is interesting to find that fossils apparently belonging to the Phyllocarida are found far earlier than any of the other Malacostraca.

In the Cambrian, and more abundantly in the Ordovician and Silurian, there are found Crustacea (Fig. 82) that resemble _Nebalia_ in having a large bivalved carapace, with a movable beak-like plate in front, a projecting abdomen without conspicuous limbs, and a pair of large spines at the sides of the telson. Unfortunately, we have almost no knowledge of the structure of the limbs; but it can hardly be doubted that these very ancient Crustacea were allied to the existing Phyllocarida, and that they included the forerunners of the higher Malacostraca.

[Ill.u.s.tration: FIG. 82--_Ceratiocaris papilio_, ONE OF THE FOSSIL PHYLLOCARIDA. (From Lankester's "Treatise on Zoology," after H.

Woodward.)

_a_, Traces of antennules; _m_, possibly mandibles; _r_, rostral plate]

It is in the Carboniferous epoch, in all probability, that we must look for the origin of most of the existing orders of Malacostraca. In the rocks of this age in different parts of the world there have been found a number of undoubted Malacostraca, nearly all of the shrimp-like form which there is good reason to believe to be a primitive characteristic.

Some of these (_Pygocephalus_--Fig. 83) have recently been shown to possess a brood-pouch formed of overlapping plates on the under-side of the thorax, and thus resemble the existing Mysidacea, which stand at the base of the Peracaridan series of orders. Others have a pair of strong side-spines near the tip of the telson, and in other ways resemble the recent Euphausiacea, so that they may have been primitive members of the Eucaridan series.

[Ill.u.s.tration: FIG. 83--_Pygocephalus cooperi_, FROM THE COAL-MEASURES: UNDER-SIDE OF A FEMALE SPECIMEN, SHOWING THE OVERLAPPING PLATES OF THE BROOD-POUCH. (From Lankester's "Treatise on Zoology," after H.

Woodward.)]

[Ill.u.s.tration: FIG. 84--THE TASMANIAN "MOUNTAIN SHRIMP" (_Anaspides tasmaniae_), A LIVING REPRESENTATIVE OF THE SYNCARIDA. SLIGHTLY ENLARGED

_c.gr._, "Cervical groove," marking off the first thoracic somite; ii-viii, the remaining thoracic somites; 1-6, the abdominal somites]

Among the Crustacea of the Carboniferous and Permian epochs, there are a number of forms of which the affinities were until recently quite obscure. They have two-branched antennules, a scale-like exopodite on the antenna, and the last pair of appendages (uropods) form, with the telson, a tail-fan. In these points they resemble the shrimp-like forms, but there is no carapace, and all the somites of the thorax are distinct, so that the form of the body is rather that of an Amphipod or Isopod. On the discovery of the remarkable Crustacean _Anaspides_ (Fig.

84), which lives in fresh-water pools in the mountains of Tasmania, it was pointed out that it agreed with the fossil genera _Uronectes_, _Palaeocaris_, and their allies, in those very characters in which they differed from all other Crustacea, and that it must be regarded as a surviving representative of the ancient group to which the name of Syncarida had been given. The more recent discoveries of living forms, _Paranaspides_ from the Great Lake of Tasmania and _Koonunga_ from fresh-water pools near Melbourne, and of the fossil _Praeanaspides_ (Fig. 85) from the Coal-measures of Derbys.h.i.+re, have tended to support this conclusion. There can be little doubt that the Syncarida arose during the Carboniferous epoch (or earlier) from primitive shrimp-like forms which lost the carapace; but, after flouris.h.i.+ng for a relatively brief period, the group dwindled away, although a few survivors have lingered on, like so many other "living fossils," in the isolated Australian region.

[Ill.u.s.tration: FIG. 85--_Praeanaspides praecursor_, ONE OF THE FOSSIL SYNCARIDA, FROM THE COAL-MEASURES OF DERBYs.h.i.+RE. SLIGHTLY ENLARGED.

(After H. Woodward.)]

It must be pointed out that, in spite of the resemblance of the body of _Anaspides_ to that of an Amphipod, the Syncarida can have had no close relation to the origin of the Isopoda and Amphipoda. These have also been derived from a shrimp-like type, but their possession of a brood-pouch, among other characters, shows that they are linked on to the Mysidacea, and must have arisen from some primitive member of that group, like _Pygocephalus_. Although palaeontology as yet gives little help in tracing the course of their evolution, we can imagine what the intermediate links must have been like by comparison with the living c.u.macea and Tanaidacea.

It is possible, indeed, that the divergence of the Isopod line of descent from that of the Mysidacea took place earlier than the Carboniferous epoch, for there has recently been discovered in rocks of Devonian age in Ireland a single specimen of a fossil, to which the name of _Oxyuropoda_ has been given, which has every appearance of being an Isopod. At all events, undoubted Isopods make their appearance in rocks of the Secondary Period, and some of those from the Jura.s.sic epoch are not very different in general form from types still existing.

Some of the Carboniferous shrimp-like Crustacea present characters which seem to point in the direction of the Stomatopoda, and fossils which clearly belong to that group are found in Jura.s.sic and later deposits.

In the Cretaceous epoch there were Stomatopoda resembling modern types so closely that they have been referred to the existing genus _Squilla_.

We are even able to say that they resembled the living Stomatopoda in their mode of development, for larvae of the type known as _Erichthus_ have been recognized in rocks of Cretaceous age from Lebanon. This is a striking example of the way in which, by a fortunate accident as it were, organisms apparently ill-adapted for fossilization may occasionally be preserved.