The Life of Crustacea Part 7

It has already been mentioned that the animals living on the sea-bottom in shallow water do not differ greatly in character from those that may be found between tide-marks. As we go farther out from land, however, into the deeper water, the character of the fauna gradually changes. One by one the species found near the sh.o.r.e become rare and disappear, and their places are taken by others characteristic of the intermediate depths. These in their turn give way to others, till in the abysses of the great oceans we find an a.s.semblage of strange animals adapted to the conditions of life in the great depths, and differing widely in many respects from the more familiar inhabitants of the coastal waters. In this "fauna of the deep sea," which extends to the greatest depths reached by the dredge or trawl, the Crustacea occupy a prominent place.

Before proceeding to discuss some of these peculiar forms, however, it is necessary to attempt to form some idea of the conditions under which they live.

In the first place, the character of the sea-bottom changes very greatly as we pa.s.s away from the coast. Near the sh.o.r.e it is extremely diversified, consisting in one place of rocks swept bare by the tides or overgrown with jungles of sea-weed, in another of banks of gravel or s.h.i.+ngle, of sand or of mud, but in all cases derived from the "waste" of the land, as it is eaten away by the waves or washed down by the rivers.

As the distance from land increases, the deposits become finer and finer, till they shade off into a soft oozy mud, composed of the finest particles brought down by the rivers. In the neighbourhood of large rivers this mud may sometimes extend for hundreds of miles from the land, but there is a limit to the distance to which even the finest particles can drift before they settle to the bottom, and beyond this limit the floor of the ocean is covered by sediments which owe their origin, not to the land, but to the ocean itself. The surface waters of the ocean everywhere teem with a vast variety of floating animals and plants, and, as these die, their remains sink to the bottom "like a perpetual shower of rain."

Among the most abundant floating organisms--in the warmer seas, at any rate--are certain minute animals known as _Foraminifera_, which belong to the lowest cla.s.s of the animal kingdom, and have sh.e.l.ls composed, in most cases, of carbonate of lime. Over vast areas the bottom of the ocean is covered with a soft grey ooze, made up almost entirely of the dead sh.e.l.ls of Foraminifera rained down from above. Since the commonest species of Foraminifera found under these circ.u.mstances belong to the genus _Globigerina_, the deposit is known as "Globigerina ooze."

In certain regions of the ocean the sh.e.l.ls of other floating organisms largely replace those of the Foraminifera in covering the ocean floor, and in the deepest abysses--so deep that the sh.e.l.ls of surface animals are dissolved before they can sink to the bottom--there is found a deposit known as the "red clay," which appears to be derived largely from the impalpable volcanic and cosmic dust that floats in the atmosphere. It is not necessary for our present purpose to enter more fully into the interesting questions connected with these deep-sea deposits, but it is important to remember that, generally speaking, the floor of the deep sea is everywhere soft ooze, without rocks or stones, except for an occasional water-logged lump of pumice or a stone dropped by a melting iceberg. This fact is probably of great importance in the life of deep-sea animals.

One of the most peculiar and characteristic of the physical conditions in the deep sea is the enormous pressure under which life has to be carried on. At the surface of the sea the pressure of the atmosphere is, roughly speaking, 14-1/2 pounds per square inch. At a depth of only 33 feet of water this pressure is doubled, and at greater depths the pressure increases in proportion, till at 2,000 fathoms it is more than 2-1/2 tons on the square inch. As a matter of fact, however, the animals at the bottom of the sea are probably but little affected by this enormous pressure. Only, when they are brought up by the dredge the sudden release of pressure causes the fluids of the body to expand and destroys the tissues, so that the animals are generally dead or dying when they reach the surface.

More important than the pressure in its influence on life is the darkness of the depths. The light of the sun only penetrates the water of the sea to a comparatively small depth. At 200 fathoms there is not enough light to produce any effect on a photographic plate. Even at a considerably less depth the absence of light puts an end to all plant-life, except for the ubiquitous bacteria, and it follows that all the animals of the deep sea ultimately depend for their food-supply on the rain of dead bodies of surface animals which, as already mentioned, is constantly falling on the sea-bottom.

The temperature at the bottom of the deep sea is always very low. Dr.

Alc.o.c.k states that "in the open part of the Bay of Bengal, where the mean surface temperature is about 80 F., the temperature at a depth of 100 fathoms is only about 60 F., at a depth of 300 fathoms not quite 50 F.; while at a depth of 2,000 fathoms the temperature all the year round is only 3 above freezing-point."

Finally, it is important to notice the _uniformity_ of the conditions at the bottom of the sea; not only are the alternation of night and day and the progress of the seasons unfelt in the abysses, but the conditions are practically the same over vast areas in all the oceans.

In the case of deep-sea Crustacea, we are frequently confronted with a difficulty which does not occur in the case of some other groups of animals--Corals or Echinoderms, for example--the difficulty, namely, of deciding whether the animals really lived on or near the bottom, or were captured by the open mouth of the trawl on its way to the surface. When the animals are plainly not well adapted for swimming--as, for instance, most of the Crabs--it may be a.s.sumed that they did actually live on the bottom; but, with the prawn-like forms, the possibility that they may really be inhabitants of the intermediate depths must always be taken into consideration.

[Ill.u.s.tration: FIG. 42--A DEEP-SEA LOBSTER (_Nephropsis stewartii_), FROM THE BAY OF BENGAL. REDUCED. (After Alc.o.c.k and Anderson.)]

[Ill.u.s.tration: FIG. 43--_Munidopsis regia_, A DEEP-SEA GALATHEID FROM THE BAY OF BENGAL. REDUCED. (After Alc.o.c.k and Anderson.)]

In animals that live in perpetual darkness we should expect to find, in accordance with the principle of adaptation which runs through the whole of organic nature, that the eyes are wanting or imperfectly developed.

In a great many deep-sea animals this is indeed the case. The deep-sea Lobsters of the genus _Nephropsis_ (Fig. 42), which are very closely allied to the Norway Lobster (_Nephrops_) of shallow water, have very short and slender eye-stalks hidden under the rostrum, and showing at the tip only the merest traces of what was once an eye. In the lobster-like Eryonidea (see Fig. 46, p. 133), the reduced eye-stalks are firmly fixed in notches in the front edge of the carapace. Some of the deep-sea Crabs and Prawns seem also to be totally blind. In a great many cases degeneration has not quite gone so far, and the eyes are present, although much reduced and modified. Thus, the very numerous deep-sea species of Galatheidae, belonging to the genus _Munidopsis_ (Fig. 43) and its allies have, as Alc.o.c.k says, "pallid, milky-yellow, lack-l.u.s.tre eyes which, though they may perhaps serve to distinguish between light and darkness, can never form a definite visual image." It is probable, indeed, that these pale-coloured eyes are specially adapted for vision in a dim light, for it has been shown that in certain deep-sea Euphausiacea the pigment-sheaths between the separate elements of the compound eyes are greatly reduced, and are fixed in the position temporarily a.s.sumed by those in the eyes of normal Crustacea when kept in the dark. Be this as it may, there are many deep-sea Crustacea which have well-developed and darkly-pigmented eyes. Some of these are swimming forms, which may at times migrate into the upper strata of water to which some rays of light penetrate; but there are some cases of Crabs and other bottom-living species that have well-developed eyes, although they live at great depths. This would seem to suggest that, although shut off from the light of day, they are not condemned to grope in perpetual darkness. Many deep-sea animals are known to be phosph.o.r.escent, and it seems probable that the large-eyed species may profit by the light emitted by the glow-worms and fireflies of the abyss. Thus, Alc.o.c.k points out that the deep-sea Hermit Crab _Parapagurus pilosima.n.u.s_ (Plate XVI.), which lives in partners.h.i.+p with a colony of Sea-anemones which it carries about with it, has large eyes, although it descends to depths of at least 2,000 fathoms; and he suggests that the Crab may be able to see its way by the light emitted by the zoophytes.

[Ill.u.s.tration: _PLATE XVI_

DEEP-SEA HERMIT-CRAB, _Parapagurus pilosima.n.u.s_, SHELTERED BY A COLONY OF _Epizoanthus_. FROM DEEP WATER OFF THE WEST OF IRELAND (SLIGHTLY REDUCED)]

Some of the Crustacea, however, are themselves luminous. Thus, Alc.o.c.k records how specimens of a deep-sea Prawn, _Heterocarpus alphonsi_, "poured out, apparently from the orifices of the 'green glands' at the base of the antennae, copious clouds of a ghostly blue light of sufficient intensity to illuminate a bucket of sea-water so that all its contents were visible in the clearest detail." Certain other Prawns are known to possess special light-producing organs on various parts of the body and limbs. It is in the Euphausiacea, however, that these organs have been most fully examined, and although the members of this group (see Fig. 24, p. 56) are by no means all deep-sea animals, some of them occurring at the surface of the sea, the structure of their luminous organs, or "photoph.o.r.es," may appropriately be described here. They are situated on the under-surface of the abdomen, in the basal segments of some of the thoracic legs, and on the upper surface of the eye-stalks.

Each consists of a globular capsule covered by a layer of pigment, except on the outer side, where there is a transparent biconvex lens. In the centre of the capsule is a peculiar "striated body" which seems to be the actual seat of luminescence, and behind it is a concave reflector composed of concentric lamellae, and having a silvery l.u.s.tre. Before their luminosity was observed, these organs were described as "accessory eyes," but there can be little doubt that they serve rather as searchlights, although, from the positions that some of them occupy on the body, it is not easy to see how they can illuminate objects within range of the eyes. That the function of phosph.o.r.escent organs is not always that of giving light for their possessor to see by is shown by the fact that many luminous animals are blind. It is important to notice, however, that these blind animals never have complex "photoph.o.r.es" like those just described, but only exhibit a diffuse luminosity or give off luminous secretions; as an example among Crustacea, the blind Eryonidea (see Fig. 46, p. 133) may be mentioned, one species of which was observed by Alc.o.c.k to be "luminous at two points between the last pair of thoracic legs, where there is a triangular glandular patch." In a recent discussion of the whole question of phosph.o.r.escence in marine organisms, Dr. Doflein concludes that the part it plays in the life of the animal probably differs in the different cases. In some it may serve to attract prey, as moths are attracted to a candle; in others it may help individuals of the same species to keep together in a swarm or to find their mates, the varying arrangement of the photoph.o.r.es producing characteristic light-patterns that serve as "recognition marks" like the colour-patterns of animals that live in the light of day. The clouds of luminous secretion thrown out by _Heterocarpus_ and other Prawns, and by certain Mysidacea and Ostracods, may serve to baffle pursuers, like the cloud of ink thrown out by a Cuttlefish, and in some cases the more complex organs may illuminate objects within the range of vision. That this does not exhaust the possibilities of speculation on the subject, however, is shown by the case of certain deep-sea Prawns which have been recently discovered to possess photoph.o.r.es placed so as to illuminate the interior of the gill cavities. What function they can discharge in this position seems beyond conjecture.

The colours of deep-sea Crustacea are very curious. Few of them have the blanched appearance common, for instance, in animals that live in the darkness of caves; on the contrary, their colours are often very vivid, but they are nearly always uniform, without spots or markings, and in a large proportion of cases are in some shade of red or orange. This red colour seems to be a.s.sociated, in some way that we do not understand, with the darkness of their habitat. The general absence of markings is very striking. Dr. Alc.o.c.k remarks that in deep-sea Crustacea we never see "those freaks of colour, or those labyrinthine mottlings and dapplings, that excite our curiosity when handling the Crabs and Shrimps of the reefs." Possibly the explanation of this may be that in these dwellers in darkness colour is merely, as it were, an accident, a by-product of physiological processes directed to other ends, not a character of protective or warning value, as in animals that hunt and are hunted in the light of day. It is a curious fact, which may have some bearing on this problem, that in many cases, while the adults are coloured in some shade of red, the eggs carried by the female are bright blue or green.

Some of the peculiarities of structure observed in deep-sea Crustacea seem to be correlated with the difficulties of resting or moving about with security on the soft ooze of the sea-floor. Among the Crabs we find a preponderance of long-legged species, not only among the true Spider Crabs (Oxyrhyncha), but also in other groups (Dromiacea like _Latreillia_, figured on Plate XIX., and Oxystomata), the members of which a.s.sume the same spider-like form. In some cases the legs are fringed with long stiff hairs, which may help to prevent the animal from sinking in the ooze, and the spines on the body and legs of many species may have the same effect. Among the deep-sea Prawns, the species of the family Nematocarcinidae (Plate XVII.) have extremely long and slender legs, which we may a.s.sume to be used like stilts for walking over the soft ooze.

[Ill.u.s.tration: _PLATE XVII_

A DEEP-SEA PRAWN, _Nematocarcinus undulatipes_. (SLIGHTLY REDUCED)

(_From Brit. Mus. Guide_)]

Not much is known regarding the food of deep-sea animals. In the absence of plant-life they must of necessity be all carnivorous, and all ultimately dependent on the food-supply falling from above. Some species have been found to have the food-ca.n.a.l filled with Globigerina ooze, which they no doubt swallow, as earth-worms do the soil in which they burrow, for the purpose of extracting the nutriment that it contains. In one species of deep-sea c.u.macea (_Platyc.u.ma holti_), which appears to feed in this manner, the food-ca.n.a.l is coiled, a condition very rare in Crustacea; in all probability this is due to the necessity for an increase of the absorptive surface, since it is common to find such an increase, either by lengthening and consequent coiling of the gut, or by infolding of its walls, in animals that have to swallow large quant.i.ties of relatively innutritious food material. Many species, however, no doubt have more selective habits of feeding. The lobster-like _Thaumastocheles_ (Fig. 44), which was dredged by the _Challenger_ expedition in the West Indies at a depth of 450 fathoms, and has since been got from deep water off the j.a.panese coast, has one of the chelae enormously enlarged, with long and slender fingers set with spines like the teeth of a rake. It has been suggested that this remarkable claw may be used for raking or sifting the ooze for small animals on which the _Thaumastocheles_ feeds. A similar function may be suggested for the long and spiny first pair of walking legs in the Spider Crab _Platymaia_ (Fig. 45).

[Ill.u.s.tration: FIG. 44--_Thaumastocheles zaleucus._ REDUCED. (After Spence Bate.)]

[Ill.u.s.tration: FIG. 45--A DEEP-SEA CRAB (_Platymaia wyville-thomsoni_).

REDUCED. (After Miers.)]

In many deep-sea Crustacea the eggs are of very large size, indicating that the young are hatched in an advanced stage of development. For example, in the numerous species of the genus _Munidopsis_ the eggs are always large and correspondingly few in number, in striking contrast to the closely allied genus _Galathea_, from shallow water, in which the eggs are small and very numerous. Alc.o.c.k mentions that a deep-sea Prawn of the genus _Psathyrocaris_, although only about 3-1/2 inches long, has eggs nearly a quarter of an inch in length. It would seem that, in some way or other, the conditions are unfavourable for a free-swimming larval life; but they cannot be altogether prohibitive, for there are a good many characteristically deep-sea Crustacea, such as the Eryonidea, that have small eggs and presumably a larval metamorphosis.

[Ill.u.s.tration: _PLATE XVIII_

_Bathynomus giganteus_, ABOUT ONE-HALF NATURAL SIZE.

(_From Lankester's "Treatise on Zoology," after Milne-Edwards and Bouvier_)]

The uniformity of the physical conditions over vast areas in the deep sea is no doubt the cause of the enormously wide geographical range of many species of deep-sea animals. There are many examples of this among Crustacea, and they are added to by every deep-sea dredging expedition.

For example, the giant Isopod _Bathynomus_ (Plate XVIII.) was first discovered in West Indian seas, and the same species has since been dredged near Ceylon, while a second species has been found off the j.a.panese coast. Of the strange lobster-like _Thaumastocheles_ (Fig.

44), mentioned above, only four specimens are known--one dredged by the _Challenger_ in the West Indies, and three others more recently brought from j.a.pan.

The low temperatures prevailing in deep water, even in tropical seas, render it possible for many Crustacea to live there which are closely allied to, or identical with, species occurring in shallow water in the colder seas of the North and South. Many examples of this are mentioned by Dr. Alc.o.c.k in his discussion of the deep-sea fauna of Indian seas; for example, the Lobster _Nephrops andamanicus_, found at depths of 150 to 400 fathoms in the Indian seas, is very closely allied to the Norway Lobster (_Nephrops norvegicus_) of our own coasts. To some extent this fact affords an explanation of the phenomenon that has been called "bipolarity" in the distribution of marine animals. It has been observed that certain families, genera, and even species, are found in the Arctic and Antarctic seas, although they seem to be entirely absent from the intervening tropical zones. In some cases, however, it has been found that these forms occur in the deep sea in the warmer regions where the cold water offers them a connection between North and South without any great difference of temperature.

In the early days of deep-sea exploration, when naturalists were becoming aware of the rich fauna inhabiting the abysses of the ocean, which till then had been supposed to be barren of all life, it was confidently expected that representatives would be discovered of some of the animals known as fossils from the earlier geological periods. It was believed that the great ocean basins had remained unchanged for vast periods of geological time, and that numerous "living fossils" would be found surviving in the depths. These hopes have not been fully realized, for the deep-sea fauna as a whole has proved to be of a comparatively modern type; nevertheless, it does include a considerable number of primitive and old-fas.h.i.+oned forms of life, some of which belong to groups elsewhere extinct. This is conspicuously the case among the Crustacea. The lobster-like Eryonidea, which at the present day are only found in the deep sea, were long known as fossils before they were discovered to survive as living animals. The existing species (Fig. 46) are all blind, with only vestiges of eye-stalks, and they may be readily distinguished by the fact that the first four, and sometimes all five, pairs of legs end in chelae, no other Decapods having more than three pairs of chelate legs. The fossils occur in rocks of the Secondary Period, from the Trias to the early Cretaceous. Some of them, at least, had well-developed eyes, and probably lived in shallow water. This was almost certainly the habitat of those (Fig. 47) that are found preserved in a marvellously perfect state in the lithographic limestone of Solenhofen (famous for the discovery of _Archaeopteryx_ and many other remarkable fossils), which is believed to have been deposited in a lagoon. After the early part of the Cretaceous epoch, the Eryonidea are no longer found as fossils, and it is, at all events, a probable conjecture that about that period they forsook the shallow waters for the deeper recesses of the ocean, where their descendants have held their own till the present day.

[Ill.u.s.tration: FIG. 46--_Polycheles phosphorus_, ONE OF THE ERYONIDEA, FEMALE, FROM THE INDIAN SEAS. (From British Museum Guide, after Alc.o.c.k.)]

[Ill.u.s.tration: FIG. 47--_Eryon propinquus_, ONE OF THE FOSSIL ERYONIDEA, FROM THE JURa.s.sIC ROCKS OF SOLENHOFEN. (From Lankester's "Treatise on Zoology," after Oppel.)]

Another group of deep-sea Crustacea which has affinities with certain fossil forms is the little family h.o.m.olodromiidae among the Crabs. It has already been mentioned that the Dromiacea are the most primitive tribe of the Brachyura, and Professor Bouvier has shown that among these the h.o.m.olodromiidae approach most nearly to the lobster-like forms from which the Crabs have been derived. He has further shown that the members of this family closely resemble in the arrangement of the grooves upon the carapace the extinct Prosoponidae, which are known as fossils from Jura.s.sic and Cretaceous rocks.

It is in the deep sea also that we find the curious Hermit Crabs of the family _Pylochelidae_ (Fig. 37, p. 94), which are perfectly symmetrical and show no trace of having ever adopted the habit of living in Gastropod sh.e.l.ls; so primitive, indeed, are these forms that it is not easy to find characters by which to define them from the lobster-like Thala.s.sinidea or from the true Lobsters themselves, and, although no fossil representatives are yet known, there seems no reason to doubt that the Pylochelidae are nearly related to the primitive stock from which the other Hermit Crabs have been evolved. Among the deep-sea Prawns there are many forms, both of Penaeidea and of Caridea, which are more primitive than most of their relatives from shallow water; and although in these cases also the geological records are faulty, we may a.s.sume, if we cannot prove in detail, a general similarity to the fossil Prawns of Mesozoic rocks.

When all has been said, however, perhaps the most surprising thing about the deep-sea fauna is, not that the animals are unlike those living in shallow water, but that they differ from them so little. When we consider the physical conditions of the oceanic abysses--the absolute darkness, the freezing cold, the pressure measured in tons on the square inch--it would seem inevitable that the physiological processes of deep-sea animals must differ greatly from those of animals living in shallow water; yet in very many cases these differences of function are accompanied only by the most trivial differences in structure. To take one example, the "Pink Shrimp" (_Pandalus montagui_), which we may find commonly between tide-marks on our own coasts, differs only in inconspicuous details from species of the same genus living at a depth of 600 fathoms; while other genera of the family Pandalidae range downwards to 2,000 fathoms or more, without any important divergences in structure.

CHAPTER VII

FLOATING CRUSTACEA OF THE OPEN SEA

It is only rarely that the floating organisms of the surface of the sea are so large or so abundant as to catch the attention of the casual observer. Except for an occasional shoal of porpoises or of flying-fish, the waste of waters seen from the deck of a s.h.i.+p in mid-ocean usually seems to be barren of life. Nevertheless, there is probably no region of the ocean where the tow-net will not reveal the existence of a more or less varied fauna and flora. Sometimes, indeed, these organisms, though minute, are so numerous as to discolour the water over large areas; whalers in the Arctic seas know by the appearance of "whale-food" where whales are likely to be found, and Herring or Mackerel fishermen recognize the changes in colour of the water among the "signs" which guide them when and where to shoot their nets.

The organisms which make up this "pelagic" fauna and flora may be grouped into two cla.s.ses, which may be termed the "swimmers," or Necton, and the "drifters," or Plankton. The former include the larger and more active animals, such as fish, whales, and the like, whose movements are more or less independent of the movements of the water; the latter comprise the plant-life and the floating or feebly swimming animals that drift at the mercy of waves and currents. A great deal of attention has been given in recent years to the study of the plankton, and it has come to be recognized as filling a very important place in the balance of life in the sea. In the sea, as on land, all the animals are ultimately dependent on plants for their food. The larger and more conspicuous sea-weeds which grow on the sea-bottom, however, can only flourish in comparatively shallow water, and the region which they occupy forms only a narrow fringe round the land-ma.s.ses of the globe. It is only necessary to look at a map of the world, showing the depth of the sea, to realize what an insignificant part of the area of the oceans contributes in this way to the food-supply of marine animals. The microscopic plant-life of the plankton, however, makes up for the individual minuteness of its const.i.tuents by their incalculable numbers.

The lowly organisms known as "diatoms," familiar to the microscopist from the beauty of their flinty skeletons, are among the most numerous and important of these, and they are a.s.sociated with a great variety of other single-celled algae and allied organisms, some of them so minute that they pa.s.s through the finest silk plankton-nets, and have to be sought for by special methods of collection recently devised for the purpose. All these organisms possess the green colouring matter (chlorophyll) that enables them to live, as the higher plants do, on the carbon dioxide and other substances dissolved in the water. The smaller animals of the plankton feed on these vegetable organisms, and in their turn serve as food for larger animals. The Herring, the Mackerel, the gigantic Basking Shark, and the still more gigantic Greenland Whale, all feed directly on the animal plankton, and we have already seen that the animals of the deep sea depend entirely on the same source of food-supply. Further, very many of the bottom-living animals of shallow water swim at the surface in the early stages of their life, and feed on the other plankton animals and plants. Indeed, it is no exaggeration to say that "all fish is diatom" in the same physiological sense as "all flesh is gra.s.s," and the study of the plankton is thus of practical importance as well as of scientific interest.

Of all the minute animals that form the intermediate links in the chain between diatom and fish or whale, the Crustacea are the most important and the most numerous both in species and in individuals. The Copepoda are more richly represented than any of the other groups, and it would be difficult to find a sample of marine plankton from which they were altogether absent. a.s.sociated with them we find one or two species of Cladocera, a larger number of Ostracoda (chiefly of the family Halocypridae), a few Mysidacea, the Amphipoda of the suborder Hyperiidea, the Euphausiacea, and some of the shrimp-like Decapods; while the larval stages of these and other groups also form an important part of the plankton.

It is necessary to make a distinction between the "neritic" plankton of shallow water near the coast and the "oceanic" plankton of the open sea.

In the insh.o.r.e waters the plankton consists not only of organisms that pa.s.s the whole of their life at or near the surface, but also, and very largely, of the free-swimming larvae of bottom-living species, and of others that make occasional and temporary excursions to the surface. For example, if the tow-net be used a short distance from land--say in some sheltered bay on our own coasts--the catch will often be found to consist largely of larval Crustacea. The zoea and megalopa stages of Crabs, the zoea and schizopod stages of Prawns and Shrimps, are often conspicuous by their numbers, or we may find swarms of the nauplius and cypris larvae of Barnacles. Sometimes, and especially at night, numbers of c.u.macea may be found in the tow-net; and it is noteworthy that these are usually males, which leave the females burrowing in the mud at the bottom, and swarm to the surface for a brief period of activity.

Besides all these more or less temporary visitors, however, there are numerous species, even in the insh.o.r.e waters, which are adapted to a floating life, and pa.s.s their whole existence as members of the plankton. Copepoda of many kinds, some Mysidae, Amphipods like _Hyperia_--which is commonly found sheltering under large jellyfish--some species of actively swimming Isopods, and many other forms, are only to be captured by the tow-net; and now and then, in certain localities, winds and currents may drive into coastal waters shoals of species whose proper home is the open ocean.

In a similar way the strictly neritic forms may sometimes be carried far out to sea, so that it is nowhere possible to draw a hard-and-fast line between the regions occupied by the neritic and the oceanic plankton.

With increasing distance from land, however, the larval stages of bottom-living species become fewer, and finally disappear altogether, and there is left an a.s.semblage of animals whose whole existence is pa.s.sed floating at the surface or at the intermediate depths. How far down from the surface this floating fauna actually descends is a question which has been much debated. It appears now to be certain that there is no stratum of water between the surface and the bottom of the ocean which is devoid of life, although the upper layers (not at, but some distance below, the surface) are probably much more densely populated than those of the abyss. Many of the species appear to undertake more or less extensive migrations in a vertical direction, coming nearer the surface at certain stages of their life-history, and sinking into deeper water at others. Further, some species at least seem to rise to the surface at night, and to sink again during the day. Apart from these vertical movements, which are as yet only imperfectly understood, it is desirable to distinguish between the "epiplankton,"

comprising the organisms which inhabit the superficial strata of the ocean down to about 100 fathoms, and the "mesoplankton," found at greater depths. The plant-life which is dependent on sunlight belongs to the epiplankton, while the animals of the mesoplankton are dependent, like the bottom animals of the deep sea, on the supply of dead food material falling from above. A third division, the "hypoplankton," has been established for those animals which live immediately above the bottom, but its distinctness from the mesoplankton has not yet been satisfactorily established. Indeed, many of the swimming forms which have already been mentioned in dealing with the Crustacea of the deep sea are probably rather to be considered as belonging to the deep mesoplankton--at least, where their size and swimming powers do not ent.i.tle them to be ranked with the "necton."