The Life of Crustacea Part 8

[Ill.u.s.tration: FIG. 48--_Conchoecia curta_, AN OSTRACOD OF THE PLANKTON.

40. (Partly after G. W. Muller.)]

Many of the modifications in structure characteristic of pelagic animals may be traced to the necessity for keeping continuously afloat with a minimum of exertion. The Crustacea of the plankton never carry the heavy armour found in bottom-living species. Thus, the thick-sh.e.l.led Ostracoda of the bottom are represented in the plankton chiefly by the family Halocypridae (Fig. 48), in which the sh.e.l.l is thin, uncalcified, and almost membranous. Many species, particularly of the Copepoda, are seen, under the microscope, to have large globules of oil distributed through the tissues of the body, and these no doubt serve as floats, increasing the buoyancy of the animal. The same purpose is probably served, in many cases, by having large s.p.a.ces, filled with fluid, within the body. This is characteristic of pelagic animals, and is well seen in many of the Crustacea in which the viscera and muscles occupy a relatively small part of the interior of the animals, the intervening s.p.a.ces being filled with colourless transparent fluid. Many of the Hyperid Amphipoda show this peculiarity--for example, the relatively gigantic _Cystisoma_, which is mesoplanktonic in deep water; and it reaches its extreme in _Mimonectes_ (Fig. 49), in which the anterior part of the body is, as it were, blown out into a balloon, giving the animal the aspect of a small jellyfish rather than an Amphipod.

[Ill.u.s.tration: FIG. 49--_Mimonectes loveni._ A FEMALE SPECIMEN SEEN FROM THE SIDE AND FROM BELOW, SHOWING THE DISTENDED-BALLOON-LIKE FORM OF THE ANTERIOR PART OF THE BODY. 3. (After Bovallius.)]

If, as seems probable, the body-fluid of these animals is of a lower specific gravity than the sea-water, it will act like the oil-globules of the Copepoda in keeping the animals afloat. Even if the specific gravity be the same, however, the distension of the body with fluid acts in another way, by increasing the surface exposed to friction with the surrounding water, and so r.e.t.a.r.ding sinking. The principle involved is ill.u.s.trated by the fact that a soap-bubble sinks much more slowly through the air than the drop of water into which it collapses. The same result is produced if the surface is increased by outstanding spines or hairs, just as, for instance, a downy feather sinks slowly through the air, but drops rapidly if it is rolled into a ball between the fingers. This is, no doubt, one function of the spines with which plankton Crustacea, and particularly larvae, are frequently provided, though they may also serve in some cases as a protection against enemies. The spines have been already alluded to in describing the various larvae, but it may be noted here that they are most strongly developed in larvae which live in the open ocean; for example, the most elaborately armed of all Decapod larvae are the zoea stages of _Sergestes_ (Fig. 50), which, like the adults, belong to the oceanic plankton. The nauplius larvae of Cirripedes are all more or less spiny, and the spines reach an exaggerated development in the larvae of the genus _Lepas_ (Fig. 51), of which the adults are attached to floating drift-wood or the like, and belong to the oceanic fauna, although hardly to be cla.s.sed with the plankton.

[Ill.u.s.tration: FIG. 50--THE ZOeA LARVA OF A SPECIES OF _Sergestes_, TAKEN BY THE "CHALLENGER" EXPEDITION. 25. (After Spence Bate.)]

[Ill.u.s.tration: FIG. 51--THE NAUPLIUS LARVA OF A SPECIES OF BARNACLE OF THE FAMILY LEPADIDae, SHOWING GREATLY-DEVELOPED SPINES. FROM A SPECIMEN TAKEN IN THE ATLANTIC OCEAN, NEAR MADEIRA. 11. (After Chun.)]

The large feathered bristles that decorate the limbs or tail of many plankton Copepoda have no doubt the same function in a.s.sisting flotation. In the genus _Calocala.n.u.s_ (Fig. 52), for example, the tail setae are large and brilliantly coloured feathery plumes, and in one species, _C. plumulosus_, one of these setae is of relatively enormous size, five or six times as long as the body of the animal itself.

[Ill.u.s.tration: FIG. 52--_Calocala.n.u.s pavo_, ONE OF THE FREE-SWIMMING COPEPODA OF THE PLANKTON. ENLARGED. (From Lankester's "Treatise on Zoology," after Giesbrecht.)]

Among the most singular of plankton Crustacea are the _Phyllosoma_ larvae (see Fig. 28, p. 72) of the Spiny Lobsters and their allies (Scyllaridea), which have been already described. These larvae are sometimes found far out at sea, and it seems likely that their larval life is unusually prolonged, and that they may be drifted to great distances by ocean currents. At all events, they are well adapted for pelagic life, since the broad flat body, hardly thicker than a sheet of paper, can be sustained in the water like a "hydroplane" by comparatively slight efforts of the swimming legs.

The watery character of the body, together with the thinness of the exoskeleton, helps to explain the gla.s.sy transparency which is a feature of most plankton Crustacea. This transparency has been regarded as a protective adaptation rendering the animals inconspicuous in the water, and it has indeed that effect to human eyes, but it is very doubtful whether the animals derive much benefit from this. Many of the animals--such as Herring and other pelagic fishes--that prey upon plankton Crustacea appear to swallow them in bulk, without much selection; and the Greenland Whale, as it swims open-mouthed through the sea, is not likely to be guided by the greater or less visibility of the Copepods that it sifts out on its baleen plates. Further, this gla.s.s-like transparency is by no means universal, for many plankton Copepoda are brightly coloured. In some, as in the beautiful blue _Anomalocera_, common in British waters, the colour is due to pigment in the fluids and tissues of the body; in others the feathery hairs on the body and limbs show brilliant metallic colours, produced, like the colours of a peac.o.c.k's feather, not by pigments, but by the diffraction of light in the texture of the organ. The most beautiful of all Copepoda is _Sapphirina_, in which the surface of the body absolutely sparkles with iridescent colours.

The striking phenomenon known as the "phosph.o.r.escence of the sea" is familiar to every ocean voyager, and is seen from time to time on our own coast. On a dark night the crest of every wave often seems to break in a pale glow, the wake of the vessel is a trail of light, and an oar dipped in the water seems on fire. This luminosity is due to the animals of the plankton, largely to the lowly Protozoa and the jellyfishes, but in part also to certain Crustacea. A number of pelagic Copepoda have been shown by Giesbrecht to secrete, from special glands on the surface of the body, a substance which becomes luminous on coming in contact with the water. Even specimens which had been dried were found to give out light on being wetted. Some pelagic Ostracods of the family Halocypridae have been observed to emit clouds of a luminous secretion from a gland in the neighbourhood of the mouth. A similar habit has been seen, as already mentioned, in certain deep-sea Prawns and Mysidacea, which may perhaps belong to the deeper part of the mesoplankton rather than to the bottom fauna. The complex light-producing organs of the Euphausiacea have already been described in dealing with deep-sea Crustacea. A great many species of this group, however, are members of the epiplankton, and in these the phosph.o.r.escent apparatus is quite as fully developed as in species coming from greater depths.

_Meganyctiphanes norvegica_ (Fig. 24, p. 56), which is one of the largest of the Euphausiacea, is common at no great depths in many places in British seas. If a jar of sea-water in which specimens of this species are swimming be brought into a dark room, a tap on the gla.s.s will cause the photoph.o.r.es to flash out like a row of tiny lamps along the side of the body. After s.h.i.+ning for a few seconds the light dies out, to appear again if the tapping be repeated.

There are certain peculiarities in the structure of the eyes in some plankton Crustacea which suggest that the sense of sight is of special importance to their possessors, although we can hardly do more than guess at their special significance. Most Copepoda have only a single eye in the middle of the head, corresponding to the single eye of the nauplius larva, and of far simpler structure than the paired compound eyes of most other Crustacea. In many plankton species, however, this simple eye becomes much enlarged and complicated in various ways. The three parts of which it is normally made up may become separated from each other, and are sometimes increased in number to five, while lenses serving to concentrate the light are often developed by thickening of the overlying cuticle. The most elaborately constructed eyes are found in the family Corycaeidae. In _Copilia_ (Fig. 53) a pair of eyes of relatively enormous size are present. Each has in front a large biconvex lens set at the end of a conical tube which extends backwards to a smaller lens (like a telescope with object-gla.s.s and eyepiece), behind which, again, are the sensory cells, corresponding to the retina, enclosed in a tube of dark pigment, the whole apparatus being more than half the length of the body. These eyes, although paired, do not correspond to the paired compound eyes of other Crustacea, but have arisen by the separation and enlargement of two of the three divisions of the typical median Copepod eye.

[Ill.u.s.tration: FIG. 53--_Copilia quadrata_ (FEMALE), A COPEPOD OF THE FAMILY CORYCaeIDae, SHOWING THE PAIR OF LARGE "TELESCOPIC" EYES. x 20.

(After Giesbrecht.)]

A peculiarity of the paired compound eyes found in plankton Crustacea of several different orders consists in the division of each eye into two parts, which differ in structure. In many Euphausiacea and Mysidacea, especially in those haunting the deeper strata (mesoplankton), this division of the eyes is well marked, a frontal or dorsal part having the separate elements of the eye (ommatidia) greatly lengthened and with reduced pigment, while the lateral part is of more normal structure. It seems probable, from the researches of Professor Chun, that the fronto-dorsal division is adapted for the perception of very faint light, while the lateral division will give a more accurate image of brightly illuminated objects.

[Ill.u.s.tration: FIG. 54--_Phronima colletti_, MALE. FROM A SPECIMEN TAKEN IN DEEP WATER NEAR THE CANARY ISLANDS. 12. (After Chun.)]

In the pelagic Amphipoda, forming the suborder Hyperiidea, the eyes are of very large size, generally occupying almost the whole surface of the head, and giving the animals a very characteristic appearance, in contrast to the small-eyed, bottom-living Gammaridea. In the family Phronimidae (Fig. 54) the eyes are each divided into two parts, differing in structure in the way just described.

There are a few Crustacea living habitually on the high seas which cannot be reckoned as belonging either to the true plankton or to the necton, since they depend on outside help for keeping themselves afloat.

Among these are the Barnacles which cl.u.s.ter on logs of drift-wood, and are among the most important causes of the "fouling" of s.h.i.+ps' hulls on long voyages. The stalked Barnacles of the genus _Lepas_ are especially common in such situations, and the characters of their larvae have been already alluded to. Certain species of sessile Barnacles are constantly found attached to large marine animals. For example, _Chelon.o.bia_ adheres to the sh.e.l.l of Turtles, while _Coronula_ and some allied genera are found on Whales.

[Ill.u.s.tration: _PLATE XIX_

_Latreillia elegans_, ONE OF THE DROMIACEA WHICH RESEMBLES A SPIDER-CRAB. FROM THE MEDITERRANEAN. (NATURAL SIZE)

THE GULF-WEED CRAB, _Planes minutus_. (SLIGHTLY ENLARGED)]

The little "Gulf-weed Crab" (_Planes minutus_--Plate XIX.) is found clinging to floating drift-weed nearly everywhere throughout the temperate and tropical seas of the globe, and is especially common in the area known as the Sarga.s.so Sea, in mid-Atlantic. It is occasionally drifted to the south coasts of the British Islands. In Sloane's "Natural History of Jamaica," published in 1707-1725, it is stated of the Gulf-weed Crab that "Columbus, finding this alive on the Sarga.s.so floating in the sea, conceived himself not far from some land, on the first voyage he made on the discovery of the West Indies."

A few other Crustacea also form part of the peculiar fauna which is a.s.sociated with the Sarga.s.so weed, notably a swimming Crab, _Neptunus sayi_, and two or three species of Prawns. All of these are coloured olive-green, like the weed among which they live.

CHAPTER VIII

CRUSTACEA OF FRESH WATERS

The Crustacean fauna of fresh water is much less rich and varied than that of the sea. Although the number of individuals in a pond or lake may be enormous, they will be found to belong to a comparatively small number of species. All the subcla.s.ses of Crustacea with the exception of the Cirripedia have representatives in fresh water, but in most of them only a very few of the families and genera comprise truly fresh-water species. In spite of the comparative poverty of the fauna, however, it is of very great interest, more especially with regard to the problems of geographical distribution; and the ease with which specimens may be collected everywhere, and kept in small aquaria, renders it a particularly attractive subject of study for the amateur naturalist.

The general uniformity of the fresh-water fauna throughout the world has often been remarked. Darwin says: "When first collecting in the fresh waters of Brazil, I well remember feeling much surprise at the similarity of the fresh-water insects, sh.e.l.ls, etc., and at the dissimilarity of the surrounding terrestrial beings, compared with those of Britain." This uniformity is well ill.u.s.trated by many of the smaller Crustacea. In a gathering of Cladocera, Copepoda, and Ostracoda, from Central Africa or from Australia, we find that most of the genera, and even some of the species, are identical with those found in similar situations in this country. It is by no means the case that all the species and genera are thus universally distributed, for there are many, especially among the larger forms, which have a very restricted range; but this does not render less striking the general uniformity of the fauna over very wide areas.

When we consider the physical environment of fresh-water animals, it seems at first sight as if this wide distribution were the reverse of what might have been expected, for the area occupied by them is far more discontinuous than in the case of terrestrial or marine animals. The inhabitants of a pond or lake are to a great extent isolated; and although they may spread to other ponds and lakes by way of communicating streams or rivers, where these are not too swiftly flowing and are not interrupted by falls, yet direct pa.s.sage from one river system to another is rarely possible. Further, since practically the whole of the fresh water on the surface of the globe is constantly flowing, more or less rapidly, towards the sea, the smaller feebly swimming forms tend to be swept down with the current, and ultimately carried to perish in the sea. It follows that only those forms which possess special adaptations for dispersal are able to flourish in fresh water. In many cases, as will be described below, the eggs of the smaller Crustacea can survive being dried up, and in this state they may be blown about by wind or carried to great distances in mud, adhering to the feet of migratory wading birds. Darwin says: "The wide-ranging power of fresh-water productions can, I think, in most cases be explained by their having become fitted, in a manner highly useful to them, for short and frequent migrations from pond to pond, or from stream to stream, within their own countries; and liability to wide dispersal would follow from this capacity as an almost necessary consequence" ("Origin of Species," sixth edition, chapter xiii.). In accordance with this, we find that it is just those groups of Crustacea which show these adaptations for dispersal that are most universally distributed in fresh water. On the other hand, the larger Crustacea, like the Crayfishes and River Crabs, which cannot so easily be transported from one locality to another, have as a rule a more restricted range. These larger forms, from their size and powers of swimming or creeping, can make their way upstream and spread throughout a river system, and in some cases they can leave the water and journey for short distances overland. On the other hand, since free-swimming larvae would be liable to be swept out to sea, most of them have a direct development, the young only leaving the protection of the mother when they have attained the form and habits of the adult. When all these factors have been taken into account, however, there still remain many cases where the distribution of individual species or of groups is hard to explain, and shows indications of dating from a time when the outlines of continents and the connections of river systems were different from what they are now.

Before proceeding to mention some of the more characteristic forms of fresh-water Crustacea, it should be mentioned that in large lakes, as in the sea, we can distinguish a littoral fauna in the shallow waters close to the sh.o.r.e, a plankton fauna of the surface waters, and a deep-water fauna. The littoral fauna does not differ in general characters from that found in smaller ponds and gently-flowing rivers; the plankton comprises many peculiar species showing adaptations for flotation, as in the case of the marine plankton; and the deep-water fauna is very poor in species and in individuals, and shows some relations with the subterranean fauna to be mentioned later.

Of all the subcla.s.ses of Crustacea, the Branchiopoda are the most characteristically fresh-water animals, only a few Cladocera being found in the sea, and some Anostraca in salt lakes and brine pools.

The larger Branchiopoda (Anostraca, Notostraca, and Conchostraca) are generally found in small, shallow ponds which are liable to be dried up in summer. The "Fairy Shrimp" (_Chirocephalus diapha.n.u.s_; see Fig. 10, p. 35) has been found in swarms in the water standing in deep cart-ruts in a country lane in England, and _Apus_ sometimes appears suddenly in rain-water puddles of a few square yards in area, which dry up after a few weeks of hot weather. The eggs of these animals, when dried in the mud, may remain dormant for long periods, and many species have been hatched out from samples of dried mud brought by travellers from distant countries. In such a sample from the Pool of Gihon at Jerusalem, it is recorded that the eggs of _Estheria_ (see Fig. 11, p. 36) were found to be capable of hatching after being kept dry for nine years. In some species it is said that the eggs will not develop unless they have been first dried, but this is not the case with _Chirocephalus_. In favourable conditions development takes place very rapidly. Messrs.

Spencer and Hall, in describing the Branchiopoda of Central Australia, say: "Certainly not more than two weeks after a fall of rain, and probably only a few days, numberless specimens of _Apus_, measuring in all about 2-1/2 to 3 inches in length, were swimming about; and, as not a single one was to be found in the water-pools prior to the rain, these must have been developed from the egg."

From what has been said, it is apparent that the larger Branchiopoda are particularly well fitted to be distributed by the agency of birds, and this is no doubt the explanation of the way in which many of the species suddenly appear in localities where they were previously unknown, and, after swarming for a longer or shorter time, sometimes for several successive seasons, as suddenly vanish. A striking example of this is afforded by _Apus cancriformis_ (see Plate II.), which formerly occurred in several localities in the South of England, and appears more or less irregularly in many parts of the Continent of Europe. No British specimens had been recorded for over forty years, and the species was believed to be extinct in this country, when it was found in 1907 by Mr.

F. Balfour Browne in a brackish marsh near Southwick, in Kirkcudbrights.h.i.+re. It can hardly be supposed that so large an animal as _Apus_, and one so easily recognized, would have escaped notice altogether had it occurred regularly in any part of the British Islands.

It is much more probable that the Scottish specimens found in 1907 had developed from eggs accidentally transported by some bird from the Continent. In 1908 a careful search in the same locality failed to reveal a solitary specimen.

The Anostraca and Notostraca usually swim with the back downwards.

Particles of mud and of animal and vegetable matter are drawn by the currents produced in swimming, into the ventral groove between the pairs of feet, and are pa.s.sed forwards to the mouth to serve as food. Some species of Conchostraca are said to swim in the same inverted position; but Messrs. Spencer and Hall, in the memoir already quoted, state that the Australian Conchostraca swim back uppermost. They attribute the difference in habit between the Conchostraca and Notostraca to the fact that in the former group the valves of the sh.e.l.l can be rapidly closed to protect the soft and vulnerable appendages, while no such protection is possible in the Notostraca. They found on one occasion a specimen of _Apus_ (Notostraca) attacked by three Water-beetles, which were tearing its soft appendages, and they suppose that _Apus_ generally escapes such attacks by swimming upside down.

The breeding habits of the Branchiopoda are also of interest, from the prevalence in many species of reproduction by unfertilized eggs, or "parthenogenesis." This may go on for many generations, and in _Apus_, for instance, it is possible to examine thousands of specimens before finding a single male, although, for some unexplained reason, males are sometimes comparatively common. It is probable that males must appear sooner or later, otherwise the series of parthenogenetic generations will come to an end; but it is not certain that this is the case, and there are some species of Conchostraca of which the males have never been seen.

[Ill.u.s.tration: FIG. 55--THE BRINE SHRIMP (_Artemia salina_). (After Sars.)

A, Female, under-side, 6; B, head of male, upper side, further enlarged, showing the large clasping antennae. The larval stages of this species are shown in Fig. 33, p. 81]

The genus _Artemia_ (Fig. 55), among the Anostraca, is peculiar in its habitat; for, while most of the Branchiopoda inhabit fresh or brackish water, it flourishes in concentrated brine. In the South of Europe it is found, as it was formerly in England, in the shallow ponds in which sea-water is exposed to evaporation for the manufacture of salt, and in these it occurs in such numbers as to give the water a reddish colour.

It is also found in salt lakes, like the Great Salt Lake of Utah, in the United States, and in many other parts of the world. The specimens from different localities often differ considerably, especially in the form of the tail-lobes; but it has been shown that these differences are more or less directly correlated with the degree of salinity of the water in which the animals live, and it is probable that the forms which have been described are all variations of a single cosmopolitan species ranging from Greenland to Australia, and from the West Indies to Central Asia. _Artemia_ is the only one of the Anostraca that is known to be parthenogenetic, some colonies consisting entirely of females, while in others males are abundant. The reddish colour above alluded to is found also in _Branchipus_, _Apus_, and other Branchiopoda, and is due, as Sir Ray Lankester first showed, to the presence in the body-fluids of haemoglobin, the red colouring matter of the blood of Vertebrates, which is important in the process of respiration.

[Ill.u.s.tration: FIG. 56--_Chydorus sphaericus_, A COMMON SPECIES OF WATER-FLEA. 50. (After Lilljeborg.)]

The smaller Branchiopoda known as "Water-fleas," forming the order Cladocera, are abundant everywhere in fresh water. _Daphnia pulex_ and other species of the genus, and the little Lynceidae, of which _Chydorus sphaericus_ (Fig. 56) is the commonest species, are to be found in ponds and ditches, and often swarm in farmyard ponds where the water is foul with decaying matter. In most gatherings from such localities only female specimens will be found, and nearly all of these will be seen to carry a cl.u.s.ter of eggs or of developing embryos in the "brood-chamber"

between the back part of the body and the sh.e.l.l. In _Daphnia pulex_ (see Fig. 12, p. 37) a single brood may consist of thirty young, and occasionally of more than twice that number. As the broods may succeed each other at intervals of two or three days, it will be seen that the multiplication of the species in favourable circ.u.mstances may be exceedingly rapid. It has been calculated that in sixty days the progeny of a single female might amount to about 13,000,000,000. In addition to these parthenogenetic eggs, which hatch at once while still within the brood-chamber, the Cladocera produce, at certain seasons, another kind of egg which requires to be fertilized by the male before it will develop. These eggs are dark in colour and are enclosed in a thick sh.e.l.l, and they do not hatch at once, but are cast off when the sh.e.l.l of the female is moulted. Very commonly these "resting eggs," as they are called, are produced in the autumn and lie dormant until the following spring, and they can survive drying or freezing without injury, while the thin-sh.e.l.led parthenogenetic eggs within the brood-chamber of the mother are easily killed. In addition to having thick sh.e.l.ls, the resting eggs are further protected in most, but not in all, cases by the moulted carapace of the parent, which is specially thickened for the purpose. This modification of the carapace is most highly developed in the family Daphniidae (Fig. 57), where a saddle-shaped area on the dorsal side, known as the "ephippium," becomes thickened, and on moulting separates from the rest of the carapace to form a compact case enclosing the two resting eggs. The outer wall of the ephippium is divided up into small hexagonal cells, which become filled with air, causing the ephippium to float at the surface of the water. In this position the ephippia readily become entangled in the feathers of birds, and in some cases the sh.e.l.l is provided with spines or hooks, which facilitate transport to other localities by such means.

[Ill.u.s.tration: FIG. 57--A WATER-FLEA, (_Daphnia pulex_), FEMALE, WITH EPHIPPIUM CONTAINING TWO "RESTING EGGS." 20. (Partly after Lilljeborg.)

The Antenna is cut short. Compare Fig. 12, p. 37.]

The appearance of males and the production of ephippial eggs--in other words, the "s.e.xual period"--is generally more or less restricted to one season of the year. In most species, particularly in those which live in lakes, the s.e.xual period occurs in the late autumn, and the ephippial eggs lie dormant during the winter, and hatch in the spring. In species living in small ponds exposed to the risk of overheating or of drying up during summer, there is often a distinct s.e.xual period in the spring, when ephippial eggs are produced to tide over the unfavourable conditions of the warmer months of the year. Although no species is known to be exclusively parthenogenetic, yet it appears that purely parthenogenetic colonies of certain species may be found in favourable localities, where they may reproduce from year to year without males ever being found.

Certain species of Cladocera belong to the plankton of lakes and large ponds, and show modifications which adapt them for a floating life. Some of these belong to the genus _Daphnia_, and differ from the species found in other situations by their gla.s.sy transparency. As in the case of many marine plankton Crustacea, this transparency is probably due to the thinness of the sh.e.l.l and to the general watery condition of the body, giving the necessary buoyancy to enable the animal to remain constantly afloat. The same effect is no doubt produced by the long terminal spine of the carapace and by the great helmet-shaped crest into which the upper part of the head is often produced. A form very characteristic of the plankton of large lakes is _Bythotrephes_ (Fig.

58), which is found in the lakes of Scotland, Ireland, Wales, and the Lake District of England. In _Bythotrephes_ the carapace does not enclose the body, but is reduced to a small brood-sac; the abdomen, however, is drawn out into a long spine, which may be two or three times as long as the body. A further point of interest is the division of the eye into a dorsal and a ventral portion, differing in structure in much the same way as do the two divisions of the eyes in certain marine plankton Crustacea (see p. 152). Another very remarkable lacustrine form is _Leptodora_, the largest of all the Cladocera, being sometimes more than half an inch in length. In this case also the carapace is very small, and does not enclose the body. The swimming antennae are very large, and the abdomen is long and divided into several segments.