[Ill.u.s.tration: FIG. 58--_Bythotrephes longima.n.u.s_, FEMALE, WITH EMBRYOS IN THE BROOD-SAC. 12. (After Lilljeborg.)]

_Leptodora_ is further remarkable on account of its mode of development.

The parthenogenetic eggs, as in other Cladocera, develop directly, but the resting eggs give rise to larvae of the nauplius type.

_Holopedium_, which is found in similar situations, surrounds itself with a ma.s.s of a jelly-like substance which it secretes. A similar envelope of jelly is found in some marine plankton animals, though not, so far as is known, in any Crustacea, and it no doubt serves to give buoyancy to the animal.

The Copepoda of fresh water are as abundant and universally distributed as the Cladocera. Species of the genus _Cyclops_ (see Fig. 14, p. 39), easily recognized by the pear-shaped body and the two egg-packets carried by the female, are to be found in almost every pond and ditch.

The genus _Canthocamptus_ comprises species of smaller size, with slender, flexible body, and carrying only a single egg-packet. The plankton of lakes and ponds includes species of _Diaptomus_ (Fig. 59), which have a narrow body and very long antennules. The latter are held out stiffly while the animal swims by rapid movements of the antennae and mouth parts, making occasional sudden leaps by means of its oar-like feet. In this genus also the egg-packet is single. The development can easily be studied by keeping egg-carrying females of _Cyclops_ in a jar of water, when the nauplius larvae will soon hatch out.

[Ill.u.s.tration: FIG. 59--_Diaptomus coeruleus_, FEMALE. 25. (After Schmeil.)]

Although the Copepoda, unlike the Cladocera, are not parthenogenetic, it has been found that certain species of _Diaptomus_ produce resting eggs capable of surviving freezing or drying. In the early part of the breeding season the eggs have thin sh.e.l.ls, and they hatch after a short time. In the autumn, however, thick-sh.e.l.led eggs are produced, which lie dormant in the mud until the following spring. It has recently been discovered that species of _Cyclops_ and _Canthocamptus_ pa.s.s through a resting stage, in which the animal surrounds itself with a coc.o.o.n-like capsule of mud held together by a glutinous secretion produced by glands on the surface of the body and limbs. The encapsuled animals, in the cases observed, lie dormant in the mud during the summer, to resume active life in the colder months of the year. It is very probable that they can also be dried without injury, and that the "coc.o.o.ns" serve the same purpose as the resting eggs of other species.

Numerous species of Ostracods, belonging to the genus _Cypris_ (see Fig.

13, B, p. 38), and other closely related genera, occur in fresh water.

Like the Cladocera, they reproduce largely by parthenogenesis, and the males of many species are rarely found, while in some species they have not yet been discovered. In Professor Weismann's laboratory at Freiburg a colony of _Cypris_ was kept in an aquarium for eight years, and during the whole of that time no males made their appearance, the colony reproducing exclusively by parthenogenesis. Probably in all species the eggs survive drying.

The common "Freshwater Shrimp" (_Gammarus pulex_), which has already been described, may be taken as a type of a large number of Amphipoda, for the most part closely allied, which are widely distributed in most regions of the world, with the exception of the tropics. _G. pulex_ itself ranges from the British Islands to Mongolia. As the eggs are carried, till they hatch, in the brood-pouch of the parent, and are not known to survive drying, it is difficult to understand in what way _Gammarus_ and its allies contrive to spread from one locality to another.

The little fresh-water Isopod _Asellus aquaticus_ (Fig. 60) is common in ponds and ca.n.a.ls in this country. It may be recognized by its general resemblance to a Woodlouse, with very long antennae, and with a pair of long, slender, forked uropods projecting behind. The species is widely distributed in Europe, and other species of the same and closely related genera are found in North America.

[Ill.u.s.tration: FIG. 60--_Asellus aquaticus_, FEMALE. 4. (After Sars.)]

In Australia and New Zealand the Isopoda are represented in fresh waters by a very peculiar group of species, forming the suborder Phreatoicidea, which have more the aspect of Amphipods than of Isopods, since the body is more or less flattened from side to side, instead of from above downwards.

With regard to the mode of distribution of the fresh-water Isopoda, there is the same difficulty as in the case of the Amphipoda, for the eggs are carried in a brood-pouch, and do not seem to be in any way protected against drought. It is no doubt in consequence of this that the fresh-water species and genera of both Amphipoda and Isopoda, though widely distributed, do not have the world-wide range of many of the more minute Crustacea described above.

The common Crayfish, _Astacus_ (or _Potamobius_) _pallipes_, is the only truly fresh-water Decapod found in England, although a small Prawn, _Palaemonetes varians_, which usually inhabits brackish water, may occasionally be found in places where the water is practically fresh.

The structure of the Crayfish is very similar to that of the Lobster, but, as already mentioned, it differs in its mode of development, having no free-swimming larval stage. From its size, and from the fact that the eggs are carried by the female, the Crayfish cannot be transported from one locality to another by the agencies which distribute the smaller fresh-water Crustacea. On the other hand, the adult animals can live out of the water for days, or even weeks, if they are kept moist, and the English species is stated to leave the water occasionally, and to make short excursions on land. Many species found in foreign countries are still more truly amphibious in their habits. It is clear, however, that the means of dispersal of the Crayfishes are very limited, and on this account the problems connected with their geographical distribution are of great interest. An admirable discussion of the subject will be found in Professor Huxley's book on the Crayfish, and the conclusions reached by him have hardly been modified by thirty years of subsequent research. Only a very brief outline can be attempted here.

[Ill.u.s.tration: FIG. 61--MAP SHOWING THE DISTRIBUTION OF CRAYFISHES.

(Partly after Ortmann.)

The dotted areas are those occupied by the Northern Crayfishes (family Astacidae). The black patches mark the areas inhabited by the Southern Crayfishes (family Parastacidae)]

Crayfishes are found in the fresh waters of the Northern and Southern Hemispheres (Fig. 61), but in each case they are practically confined to the temperate regions, and are absent from a broad intervening tropical zone. The Northern Crayfishes, forming the family Astacidae (or Potamobiidae) are distinguished, among other characters, by having a pair of appendages on the first abdominal somite, at least in the male s.e.x; the Southern Crayfishes have no appendages on that somite, and for this and other reasons are regarded as const.i.tuting a distinct family--Parastacidae. There is thus a general correspondence between the geographical distribution of the Crayfishes and the more important structural differences expressed in their cla.s.sification. There can be no doubt that the two families have been derived from a common stock of marine lobster-like animals, and it is reasonable to suppose that two branches of this stock became independently adapted to a fresh-water habitat in the North and in the South, giving rise to the Astacidae and the Parastacidae respectively.

The distribution of the individual genera is, however, not so easy to understand. The species found in Europe all belong to the genus _Astacus_, which also penetrates into Asia as far as Turkestan and the basin of the River Obi.

Throughout the greater part of Asia no Crayfishes are found until we come to the Far East, where we find an isolated colony in the river-system of the Amur, in Korea, and in the north of j.a.pan. These far eastern Crayfishes, however, differ so much from the typical species of _Astacus_ that they are now placed in a subgenus (sometimes regarded as a distinct genus), _Cambaroides_. Curiously enough, the typical genus _Astacus_ reappears again on the other side of the Pacific, where several species occur in that part of North America which lies west of the Rocky Mountains. East of the Rockies, again, numerous species are found belonging to a distinct genus, _Cambarus_, which ranges from Canada to Central America and Cuba, and this genus is allied in certain respects to the _Cambaroides_ of Eastern Asia. If the systematic relations of these genera have been properly interpreted, it is by no means easy to understand in what way their present distribution has been brought about.

[Ill.u.s.tration: _PLATE XX_

THE MURRAY RIVER "LOBSTER," _Astacopsis spinifer_. NEW SOUTH WALES.

(MUCH REDUCED)

THE LAND CRAYFISH, _Engaeus cunicularis_. TASMANIA (NATURAL SIZE)]

The Southern Crayfishes have an even more scattered and discontinuous range. In New Zealand the genus _Paranephrops_ occurs, in Australia and Tasmania the genera _Astacopsis_ (Plate XX.), _Cheraps_ and _Engaeus_ (Plate XX.). A single species of _Cheraps_ has been recorded from New Guinea, but no Crayfishes are found in any part of the Malay Archipelago, in Southern Asia, or on the continent of Africa, although, curiously enough, a single species of a peculiar genus (_Astacoides_) is found in Madagascar. In South America species of _Parastacus_ are found in Southern Brazil, Argentina, and Chili. It is evident that these various genera of Parastacidae, which are now so widely isolated from each other, must have reached their present habitats when the relative distribution of land and sea in the Southern Hemisphere was very different from what it is now. What exactly the nature of the land connection between the various islands and continents was, whether by way of an Antarctic continent or otherwise, is a question that can only be suggested here. To attempt to answer it would involve the consideration of the distribution of many other groups of animals besides Crayfishes.

Before leaving the Crayfishes, it may be mentioned that certain species have become adapted to almost terrestrial habits. A number of species of _Cambarus_ in North America are often found at considerable distances from open water, burrowing in damp earth, their burrows reaching down to the ground-water. In many cases they throw up chimney-like piles of mud at the mouths of their burrows, and in places their chimneys are so numerous as to "hamper farming operations by interfering with the harvesting machines, clogging and ruining them." The species of _Engaeus_ (Plate XX.), found in Tasmania, are there known as "Land Crabs," and burrow in marshy places and in the forests up to an elevation of 4,000 feet.

[Ill.u.s.tration: _PLATE XXI_

_Palaemon jamaicensis_, A LARGE FRESHWATER PRAWN OF THE FAMILY PALaeMONIDae. WEST INDIES. (MUCH REDUCED)]

The broad equatorial belt which separates the regions inhabited by the Northern and the Southern Crayfishes is characterized by the presence of several other groups of fresh-water Decapoda. The large River Prawns, which are found nearly everywhere within the tropics, belong to the genus _Palaemon_ (Plate XXI.), which is very closely related to the common marine Prawns (_Leander_) of our own coasts. Some of these Prawns grow to a foot or more in length of body, and the large claws may measure as much again. From the Crayfishes, for which they are sometimes mistaken, they may be easily distinguished by the fact that the large pincer-claws are not the first, but the second pair of legs. Another widely-spread group of River Prawns, for the most part of small size, is the family Atyidae (Plate XXII.), in which the two pairs of pincer-claws are feeble, and have the fingers tipped with brushes of long hairs, used in sweeping up minute particles of food from the mud. The distribution of these Prawns presents many difficult problems, as an example of which we may mention the presence of identical or closely related species in the fresh waters of West Africa and of the West Indies.

[Ill.u.s.tration: _PLATE XXII_

_Atya scabra_, A FRESH-WATER PRAWN OF THE FAMILY ATYIDae, WEST INDIES (REDUCED)]

The Brachyura (or Crabs) include many species that live in fresh water.

Some of these, like the species of _Sesarma_ (see Plate XXIII.) and some other genera of the family Grapsidae, are common throughout the tropics, pa.s.sing up the rivers from the brackish water of estuaries, and being often found long distances inland in quite fresh water. The true River Crabs, however, belong to the family Potamonidae, and are very common throughout the warmer regions of the globe. One species, _Potamon edule_ (Plate XXIII.), formerly called _Telphusa fluviatilis_, is found in the South of Europe (Italy, Greece, etc.). Very numerous species, as yet only imperfectly known, occur throughout the whole of Africa, in Southern Asia, and in the Malay Islands, extending to Australia in the south and j.a.pan on the north. In the New World the River Crabs are found in South America, and extend north to Mexico and the West Indian Islands. Many of the River Crabs are amphibious in habits, and may be found burrowing in marshy ground or in damp forests. The young are hatched from the egg with all the appendages developed, and they remain clinging to the abdomen of the mother until after the first moult, when they are perfectly-formed little Crabs (see Fig. 31, p. 78).

[Ill.u.s.tration: _PLATE XXIII_

THE RIVER-CRAB OF SOUTHERN EUROPE, _Potamon edule_ (OR _Telphusa fluviatilis_) (REDUCED)

_Sesarma chiragra_, A FRESHWATER CRAB OF FAMILY GRAPSIDae. FROM BRAZIL. (SLIGHTLY REDUCED)]

The groups which have been mentioned are all characteristic inhabitants of the fresh waters over considerable areas of the surface of the globe.

There are, however, in addition to these, certain Crustacea which occur in isolated localities, and have no close allies in fresh waters elsewhere. In the streams of Southern Brazil and Chili there is found a small Crustacean (_aeglea laevis_--Plate XXIV.), not unlike the Galatheas of our own coasts, which is interesting as being the only species of the Anomura found in fresh water. Still more remarkable are the Syncarida, which are represented by two species of "Mountain Shrimps" (see Fig. 84, p. 264) in Tasmania, and by a third species found near Melbourne. These forms have no near allies among living Crustacea, but appear to be related, as will be shown in a later chapter, to certain fossil Crustacea found in Palaeozoic rocks.

[Ill.u.s.tration: _PLATE XXIV_

_aeglea laevis._ SOUTH AMERICA. (NATURAL SIZE)]

Belonging to a different category from any of those mentioned are certain Crustacea closely allied to, or identical with, species living in the sea, which inhabit inland lakes where no direct pa.s.sage from the sea is now possible. Attention was first called to these in the case of some of the large lakes of Sweden, in which Professor Loven found some Crustacea--_Mysis relicta_ (see Fig. 16, p. 47), _Mesidotea entomon, Pontoporeia affinis_--almost or quite identical with species inhabiting the Baltic, the Arctic Ocean, and the North Atlantic. There is geological evidence to show that these lakes were once fjords, or arms of the sea, and have become cut off from communication with the Baltic by gradual elevation of the land. The marine animals which they contained would thus be imprisoned, and as the water became less and less salt, by the inflow of rivers, certain species which were able to accommodate themselves to the altered conditions would survive. Some of the species living in the Swedish lakes have since been found to have a wider distribution. Thus, _Mysis relicta_, which should perhaps be reckoned as only a variety of the _Mysis oculata_ of Arctic seas, has been found in lakes in Russia, North Germany, and North America (Lake Superior and others), and has lately been discovered in Lough Neagh and some other lakes in Ireland.

The brackish waters of the Caspian Sea contain a very remarkable a.s.semblage of animals, including many Crustacea, which, although now quite isolated from the oceans, are certainly of marine, and in part of Arctic, origin. Among these are some species closely allied to or identical with those of the Swedish lakes already mentioned, together with a great variety of species of Mysidacea, c.u.macea, and Amphipoda, which appear to have been evolved from marine forms since the Caspian was cut off from communication with the Arctic Ocean.

To such a.s.semblages of animals derived from marine species and isolated in inland lakes the name of "relict" faunas has been given. It is necessary to use caution, however, in extending this explanation of their origin to every case of peculiar lake faunas. For example, there are difficulties in the way of supposing that Lake Baikal was ever in open and direct communication with the sea, although it contains many animals, such as seals, which are certainly of marine origin. The chief Crustacea of the lake are numerous species of Amphipods belonging to the genus _Gammarus_, and other genera closely related thereto, and for these, at all events, there is no need to a.s.sume a "relict" origin.

One of the most remarkable lakes in the world from a zoological point of view is Lake Tanganyika in Africa. When it was found that this lake contained a fauna very different from that of the other great lakes of Africa, it was rashly a.s.sumed that it must be of relict origin, and some remarkable speculations were indulged in as to the former connection between the lake and the sea. Further research, while it has greatly emphasized the peculiar nature of the fauna, has entirely disposed of the view that it originated in this way. The Crabs and Prawns, for example, are not nearly related to marine forms, but belong to groups that are characteristic of fresh waters in the tropics. While Nya.s.sa and the Victoria Nyanza have as yet only yielded a single species of Prawn, and that one of enormously wide distribution (from the Nile to Queensland), Tanganyika contains no fewer than twelve species, all of which are peculiar to the lake, while all except one belong to genera unrepresented elsewhere. Similarly, the Crabs found in the other great lakes of Africa belong to commonplace types of River Crabs of the genus _Potamon_; in Tanganyika, in addition to some of these, there are three species of a remarkable genus, _Platytelphusa_, not known from any other locality. The Copepoda and Ostracoda of Tanganyika comprise a remarkably large number of species, many of them peculiar to the lake. A most unusual feature is the entire absence of Cladocera. It is not easy to explain the occurrence of this remarkable fauna in Tanganyika, but the evidence from other groups of animals, such as Mollusca and fishes, tends to suggest that the lake must have been, until recently, completely isolated from the other lakes and river-systems of Africa, that it had no outlet, and that the water was consequently more or less brackish. Under these conditions the fauna of the lake, originally similar to that of the other African lakes, has evolved along lines of its own.

[Ill.u.s.tration: FIG. 62--A WELL SHRIMP (_Niphargus aquilex_). 7. (After Wrzesniowski.)]

A very interesting division of the fresh-water fauna is const.i.tuted by those animals which inhabit underground waters. In the South of England there is found not unfrequently in the water of wells a small colourless transparent Amphipod known as the "Well Shrimp" (_Niphargus aquilex_--Fig. 62), distinguished from the common fresh-water _Gammarus_ by the slenderness of its body, by the elongation of the last pair of tail appendages (uropods), and by the absence of eyes. The proper habitat of _Niphargus_ is not actually in the wells, but in the subterranean reservoirs and streams by which the wells are fed. These subterranean channels intercommunicate over wide areas, and are now known in many parts of the world to contain a peculiar a.s.semblage of animals which become accessible to the naturalist in wells and in the streams and lakes of large caves. Further, the scanty "abyssal" fauna of deep lakes is partly made up of species which enter the lakes by subterranean channels, and find a suitable habitat in the deep water.

Species of _Niphargus_, for example, have been dredged in Lough Mask in Ireland and in some of the Swiss lakes.

Several species of blind Crayfishes have been found in caves in North America, the best known being one (_Cambarus pellucidus_--Plate XXV.) found in the Mammoth Cave in Kentucky; and blind Prawns belonging to various genera have been discovered in caves in America and Europe.

[Ill.u.s.tration: _PLATE XXV_

THE BLIND CRAYFISH OF THE MAMMOTH CAVE OF KENTUCKY, _Cambarus pellucidus_. (NATURAL SIZE)]

A very remarkable feature of the subterranean fauna is that a number of the animals appear to be more closely allied to marine species than to any known from fresh waters above-ground. This is especially the case with some of the Isopoda belonging to typically marine families like the Cirolanidae and Anthuridae, and it has been suggested that these have been derived from marine species which have entered the underground waters directly from the sea by way of submarine fissures in the crust of the earth.