The Brine ShrimpFig. 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 antennæ. The larval stages of this species are shown inFig. 33, p. 81View larger image
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 antennæ. The larval stages of this species are shown inFig. 33, p. 81
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Chydorus sphæricusFig. 56—Chydorus sphæricus,a Common Species of Water-flea.× 50. (After Lilljeborg.)View larger image
Fig. 56—Chydorus sphæricus,a Common Species of Water-flea.× 50. (After Lilljeborg.)
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The genusArtemia(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 exposedto 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.Artemiais 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 inBranchipus,Apus, and other Branchiopoda, and is due, as Sir Ray Lankester first showed, to the presence in the body-fluids of hæmoglobin, the red colouring matter of the blood of Vertebrates, which is important in the process of respiration.
A Water-fleaFig. 57—A Water-flea, (Daphnia pulex),Female, with Ephippium containing Two "Resting Eggs."× 20. (Partly after Lilljeborg.)The Antenna is cut short. CompareFig. 12, p. 37.View larger image
Fig. 57—A Water-flea, (Daphnia pulex),Female, with Ephippium containing Two "Resting Eggs."× 20. (Partly after Lilljeborg.)
The Antenna is cut short. CompareFig. 12, p. 37.
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The smaller Branchiopoda known as "Water-fleas," forming the order Cladocera, are abundant everywhere in fresh water.Daphnia pulexand otherspeciesof the genus, and the little Lynceidæ, of whichChydorus sphæricus(Fig. 56) is the commonest species, are to be found in ponds and ditches, andoften 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 cluster of eggs or of developing embryos in the "brood-chamber" between the back part of the body and the shell. InDaphnia pulex(seeFig. 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 circumstances 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 shell, and they do not hatch at once, but are cast off when the shell 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 cansurvive drying or freezing without injury, while the thin-shelled parthenogenetic eggs within the brood-chamber of the mother are easily killed. In addition to having thick shells, 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 Daphniidæ (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 shell is provided with spines or hooks, which facilitate transport to other localities by such means.
The appearance of males and the production of ephippial eggs—in other words, the "sexual period"—is generally more or less restricted to one seasonof the year. In most species, particularly in those which live in lakes, the sexual 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 sexual 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 genusDaphnia, and differ from the species found in other situations by their glassy transparency. As in the case of many marine plankton Crustacea, this transparency is probably due to the thinness of the shell 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 isBythotrephes(Fig. 58), which is found in the lakes of Scotland, Ireland, Wales, and the Lake District of England. InBythotrephesthe carapace does not enclose the body, but is reduced to a small brood-sac; theabdomen, 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 (seep. 152). Another very remarkable lacustrine form isLeptodora, 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 antennæ are very large, and the abdomen is long and divided into several segments.
Bythotrephes longimanusFig. 58—Bythotrephes longimanus,Female, with Embryos in the Brood-sac. × 12. (After Lilljeborg.)View larger image
Fig. 58—Bythotrephes longimanus,Female, with Embryos in the Brood-sac. × 12. (After Lilljeborg.)
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Leptodorais 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 larvæ of the nauplius type.
Holopedium, which is found in similar situations, surrounds itself with a mass 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 genusCyclops(seeFig. 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 genusCanthocamptuscomprises species of smaller size, with slender, flexible body, and carrying only a single egg-packet. The plankton of lakes and ponds includes species ofDiaptomus(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 antennæ 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 ofCyclopsin a jar of water, when the nauplius larvæ will soon hatch out.
Diaptomus coeruleusFig. 59—Diaptomus cœruleus,Female. × 25. (After Schmeil.)View larger image
Fig. 59—Diaptomus cœruleus,Female. × 25. (After Schmeil.)
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Although the Copepoda, unlike the Cladocera, are not parthenogenetic, it has been found that certain species ofDiaptomusproduce resting eggs capable ofsurviving freezing or drying. In the early part of the breeding season the eggs have thin shells, and they hatch after a short time. In the autumn, however, thick-shelled eggs are produced, which lie dormant in the mud until the following spring. It has recently been discovered that species ofCyclopsandCanthocamptuspass through a resting stage, in which the animal surrounds itself with a cocoon-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 "cocoons"serve the same purpose as the resting eggs of other species.
Numerous species of Ostracods, belonging to the genusCypris(seeFig. 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 ofCypriswas 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. pulexitself 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 wayGammarusand its allies contrive to spread from one locality to another.
The little fresh-water IsopodAsellus aquaticus(Fig. 60) is common in ponds and canals in this country. It may be recognized by its general resemblanceto a Woodlouse, with very long antennæ, 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.
Asellus aquaticusFig. 60—Asellus aquaticus,Female. × 4. (After Sars.)View larger image
Fig. 60—Asellus aquaticus,Female. × 4. (After Sars.)
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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 arecarried 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(orPotamobius)pallipes, is the only truly fresh-water Decapod found in England, although a small Prawn,Palæmonetes 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.
Map showing the Distribution of CrayfishesFig. 61—Map showing the Distribution of Crayfishes.(Partly after Ortmann.)The dotted areas are those occupied by the Northern Crayfishes (family Astacidæ). The black patches mark the areas inhabited by the Southern Crayfishes (family Parastacidæ)View larger image
Fig. 61—Map showing the Distribution of Crayfishes.(Partly after Ortmann.)
The dotted areas are those occupied by the Northern Crayfishes (family Astacidæ). The black patches mark the areas inhabited by the Southern Crayfishes (family Parastacidæ)
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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 Astacidæ (or Potamobiidæ) are distinguished, among other characters, by having a pair of appendages on the first abdominal somite, at least in the male sex; the Southern Crayfishes have no appendages on that somite, and for this and other reasons are regarded as constituting a distinct family—Parastacidæ. There is thus a general correspondence between the geographical distribution of the Crayfishes and the more important structural differences expressed in their classification. 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 Astacidæ and the Parastacidæ respectively.
The distribution of the individual genera is, however, not so easy tounderstand. The species found in Europe all belong to the genusAstacus, 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 Japan. These far eastern Crayfishes, however, differ so much from the typical species ofAstacusthat they are now placed in a subgenus (sometimes regarded as a distinct genus),Cambaroides. Curiously enough, the typical genusAstacusreappears 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 theCambaroidesof 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.
PLATE XXThe Murray River LobsterTHE MURRAY RIVER "LOBSTER,"Astacopsis spinifer.NEW SOUTH WALES.(MUCH REDUCED)View larger imageThe Land CrayfishTHE LAND CRAYFISH,Engæus cunicularis.TASMANIA(NATURAL SIZE)View larger image
PLATE XX
The Murray River LobsterTHE MURRAY RIVER "LOBSTER,"Astacopsis spinifer.NEW SOUTH WALES.(MUCH REDUCED)View larger image
THE MURRAY RIVER "LOBSTER,"Astacopsis spinifer.NEW SOUTH WALES.(MUCH REDUCED)
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The Land CrayfishTHE LAND CRAYFISH,Engæus cunicularis.TASMANIA(NATURAL SIZE)View larger image
THE LAND CRAYFISH,Engæus cunicularis.TASMANIA(NATURAL SIZE)
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The Southern Crayfishes have an even more scattered and discontinuous range. In New Zealand the genusParanephropsoccurs, in Australia and Tasmania the generaAstacopsis(Plate XX.),CherapsandEngæus(Plate XX.). A single species ofCherapshas been recordedfrom 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 ofParastacusare found in Southern Brazil, Argentina, and Chili. It is evident that these various genera of Parastacidæ, 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 ofCambarusin 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 ofEngæus(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.
Palæmon jamaicensisPLATE XXIPalæmon jamaicensis,A LARGE FRESHWATER PRAWN OF THE FAMILY PALÆMONIDÆ. WEST INDIES.(MUCH REDUCED)View larger image
PLATE XXI
Palæmon jamaicensis,A LARGE FRESHWATER PRAWN OF THE FAMILY PALÆMONIDÆ. WEST INDIES.(MUCH REDUCED)
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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 genusPalæmon(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 Atyidæ (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.
Atya scabraPLATE XXIIAtya scabra,A FRESH-WATER PRAWN OF THE FAMILY ATYIDÆ, WEST INDIES(REDUCED)View larger image
PLATE XXII
Atya scabra,A FRESH-WATER PRAWN OF THE FAMILY ATYIDÆ, WEST INDIES(REDUCED)
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The Brachyura (or Crabs) include many species that livein fresh water. Some of these, like the species ofSesarma(seePlate XXIII.) and some other genera of the family Grapsidæ, are common throughout the tropics, passing 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 Potamonidæ, and are very common throughout the warmer regions of the globe. One species,Potamon edule(Plate XXIII.), formerly calledTelphusa 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 Japan 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 (seeFig. 31, p. 78).
PLATE XXIIIPotamon eduleTHE RIVER-CRAB OF SOUTHERN EUROPE,Potamon edule(ORTelphusa fluviatilis) (REDUCED)View larger imageSesarma chiragraSesarma chiragra,A FRESHWATER CRAB OF FAMILY GRAPSIDÆ. FROM BRAZIL.(SLIGHTLY REDUCED)View larger image
PLATE XXIII
Potamon eduleTHE RIVER-CRAB OF SOUTHERN EUROPE,Potamon edule(ORTelphusa fluviatilis) (REDUCED)View larger image
THE RIVER-CRAB OF SOUTHERN EUROPE,Potamon edule(ORTelphusa fluviatilis) (REDUCED)
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Sesarma chiragraSesarma chiragra,A FRESHWATER CRAB OF FAMILY GRAPSIDÆ. FROM BRAZIL.(SLIGHTLY REDUCED)View larger image
Sesarma chiragra,A FRESHWATER CRAB OF FAMILY GRAPSIDÆ. FROM BRAZIL.(SLIGHTLY REDUCED)
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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 waterselsewhere. In the streams of Southern Brazil and Chili there is found a small Crustacean (Æglea lævis—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" (seeFig. 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 Palæozoic rocks.
Æglea lævisPLATE XXIVÆglea lævis.SOUTH AMERICA.(NATURAL SIZE)View larger image
PLATE XXIV
Æglea lævis.SOUTH AMERICA.(NATURAL SIZE)
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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 passage 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 Lovén found some Crustacea—Mysis relicta(seeFig. 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 becameless 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 theMysis oculataof 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 assemblage 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, Cumacea, 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 assemblages 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 anddirect 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 genusGammarus, and other genera closely related thereto, and for these, at all events, there is no need to assume 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 assumed 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 Nyassa 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 thegenusPotamon; 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.
A Well ShrimpFig. 62—A Well Shrimp(Niphargus aquilex). × 7. (After Wrzesniowski.)View larger image
Fig. 62—A Well Shrimp(Niphargus aquilex). × 7. (After Wrzesniowski.)
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A very interesting division of the fresh-water fauna is constituted 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-waterGammarusby 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 ofNiphargusis not actually in the wells, but in the subterranean reservoirs and streams by which the wells are fed. Thesesubterranean channels intercommunicate over wide areas, and are now known in many parts of the world to contain a peculiar assemblage 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 ofNiphargus, 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 tovarious genera have been discovered in caves in America and Europe.
Blind CrayfishPLATE XXVTHE BLIND CRAYFISH OF THE MAMMOTH CAVE OF KENTUCKY,Cambarus pellucidus. (NATURAL SIZE)View larger image
PLATE XXV
THE BLIND CRAYFISH OF THE MAMMOTH CAVE OF KENTUCKY,Cambarus pellucidus. (NATURAL SIZE)
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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 Cirolanidæ and Anthuridæ, 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.
The environment in which these subterranean animals live resembles that of the deep-sea animals in the absence of light, and the consequent absence of plant-life. They must ultimately depend for food on animal and vegetable débris washed down from the surface, but the food-supply must be scanty, for the water in which they live is usually very clear and free from organic matter. It is not surprising to find that nearly all of them are blind, and the few species provided with visual organs which have been described, from caves, are probably only temporary or accidental immigrants. Whether the degeneration of the eyes is the direct effect of disuse, or is due to natural selection ceasing to keep the eyes up to the standard of usefulness, is a question which has been much debated, and its answer, were we sure of it, would settle someof the most fundamental problems of the evolution theory.
At all events, we do not find in any truly subterranean species large and peculiarly modified eyes like those of many deep-sea animals, and this may be associated with the complete darkness of their habitat, not lighted by phosphorescent organisms as the deep sea is. In another respect these animals differ from those of the deep sea, for they are all colourless or nearly so; while many of the inhabitants of the deep sea, as we have already seen, are brilliantly coloured.
There is every reason to believe that the Arthropoda, like the other great groups of the animal kingdom, had their origin in the sea; but they must have invaded the dry land at a very early period, and most of the classes into which the group is divided—the Arachnids, Myriopods, and Insects—are now predominantly terrestrial in their habits. The Crustacea alone have remained for the most part aquatic animals, and only in a comparatively few cases have they succeeded in adapting themselves completely to an air-breathing existence. As already mentioned, a considerable number, both of marine and of fresh-water species, are more or less amphibious in their habits. Thus, the common Shore Crab of our own coasts and the Grapsoid Shore Crabs of warmer seas voluntarily leave the water and scramble about among the rocks between, and even above, tide-marks. Some Crabs, likeOcypodeandGelasimus(seePlate XV.), have gone farther towards becoming land-dwellers, since theirgill chambers are adapted to serve as lungs for breathing air, and some species may even be drowned by keeping them in water. The marsh-dwelling or fresh-water Crabs of the genusSesarma(seePlate XXIII.) and allied genera are also apparently to some extent air-breathers, and one species,Aratus pisonii, is stated by Fritz Müller to climb mangrove bushes and to feed on their leaves. Some Crayfishes, like theEngæusof Tasmania (seePlate XX.), already mentioned, are practically land animals. Finally, some Amphipoda, closely allied to the Sand-hoppers of British coasts, live in damp places on land, although they do not show any conspicuous modifications of structure to adapt them to this mode of life. Of one of these Amphipoda,Talitrus sylvaticus, Mr. G. Smith writes: "This species of land-hopper is widely distributed in the highlands of Tasmania, being found under logs and leaves in the forests on Mount Wellington, and in very great abundance in the beech-forests on the mountains of the west coast."
It will thus be seen that it is impossible to draw any sharp distinction between aquatic and terrestrial Crustacea, and it is chiefly from motives of convenience that we have left to be dealt with in this chapter three groups of land-dwelling Crustacea—the Land Crabs of the family Gecarcinidæ, the Land Hermits (Cœnobitidæ), and the Land Isopods, or Woodlice (Oniscoidea).
PLATE XXVIGecarcinus ruricolaA WEST INDIAN LAND-CRAB,Gecarcinus ruricola. (REDUCED)View larger imageCoenobita rugosaA LAND HERMIT CRAB,Cœnobita rugosa. (REDUCED)View larger image
PLATE XXVI
Gecarcinus ruricolaA WEST INDIAN LAND-CRAB,Gecarcinus ruricola. (REDUCED)View larger image
A WEST INDIAN LAND-CRAB,Gecarcinus ruricola. (REDUCED)
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Coenobita rugosaA LAND HERMIT CRAB,Cœnobita rugosa. (REDUCED)View larger image
A LAND HERMIT CRAB,Cœnobita rugosa. (REDUCED)
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The Gecarcinidæ are abundantin the tropics of the Old and New Worlds. Some of the species at least, probably all, visit the sea at intervals for the purpose of hatching off the eggs carried by the females, and the larval stages are passed in the sea. In the case ofGecarcinus ruricola(Plate XXVI.), a species very common in the West Indies, the migration to the sea takes place annually during the rainy season in May. The Crabs are described as coming down from the hills in vast multitudes, clambering over any obstacles in their way, and even invading houses, in their march towards the sea. Stebbing states that "The noise of their march is compared to the rattling of the armour of a regiment of cuirassiers." The females enter the sea to wash off the eggs which they carry attached to their abdominal appendages, or rather, probably, to allow the young to hatch out. The Crabs then return whence they came, and are followed later by the young, which, having passed through their larval stages in the sea, leave the water, and are found in thousands clinging to the rocks on the shore.
On Christmas Island, in the Indian Ocean, Dr. C. W. Andrews studied the habits of another Land Crab, of which the proper name seems to beGecarcoidea lalandii. He says: "This is the commonest of the Land Crabs inhabiting the island, and is found in great numbers everywhere, even on the higher hills and the more central portion of the plateau.In many places the soil is honeycombed by its burrows, into which it rapidly retreats when alarmed. These Crabs seem to feed mainly on dead leaves, which they carry in one claw held high over the back and drag down into the burrows. From their enormous numbers, they must play a great part in the destruction of decaying vegetable matter and its incorporation into the soil."
"Once a year, during the rainy season, they descend to the sea to deposit [or, rather, to hatch out] their eggs, and during this migration hundreds may be seen on every path down steep slopes, and many descend the cliff-face itself. They remain on the beach for a week or two, and ... afterwards gradually make their way back to their accustomed homes."
In the year of Dr. Andrews' first visit to the island (1898) this migration occurred in January. On a subsequent visit to the island in 1908 he obtained specimens of a large Megalopa larva (seep. 70) which occurred in enormous quantities in the sea shortly after the migration, and also of a small Crab which appeared in similar numbers at a slightly later date. It seems practically certain that these larvæ and young are those ofGecarcoidea lalandii. A second species of Land Crab,Cardisoma hirtipes, found on Christmas Island, has very different habits from the foregoing. Dr. Andrews says of it: "In this island, at any rate, this species must be regarded as a fresh-water form, and, in fact, when a specimenwas seen it might be taken as an indication that fresh water was not far off. It lives in deep holes in the mud at the sides and bottom of the brooks." Dr. Andrews tells me that he never saw this species at or near the sea (in marked contrast toGecarcoidea), and this agrees with the observations of other travellers on species of the genusCardisoma, so that the breeding habits remain unknown. There is every probability, however, that in this case, also, the young stages are passed in the sea.
The student will find, in many textbooks on zoology, the statement that some Land Crabs of the genusGecarcinusdevelop without metamorphosis. Although it is impossible, with our present knowledge, to state definitely that this is not the case, there is absolutely no evidence to support it, and it is an interesting example of the way in which erroneous statements sometimes gain currency in science.[3]It is based upon the fact that in 1835 Professor J. O. Westwood described the early stages of "a West Indian Land Crab," in a paper "On the Supposed Existence of Metamorphosis in the Crustacea," published in the Transactions of the Royal Society. Professor Westwood found that the embryos extracted from the egg possessed all the appendages of the adult except the swimmerets, and that young specimens clinging to the abdomen ofthe parent were perfectly-formed little Crabs. The specimens which he described were sent to him by the Rev. Lansdown Guilding, of St. Vincent, who also deals with the subject in a note published in theMagazine of Natural Historyin the same year. Neither Westwood nor Guilding refers to the Crab as aGecarcinus, although Guilding calls it the "Mountain Crab," a name which Patrick Browne in 1756 gives to theGecarcinus ruricolaof Jamaica. So far as I am aware, the first writer to refer to Westwood's Crab as aGecarcinus, was Professor T. Bell, who in his "British Stalk-eyed Crustacea," published in 1853, states that some of the original specimens had come into his possession. They consisted of the detached abdomens of female Crabs, with eggs and young adhering to them. It would be by no means easy to identify the species of Crab to which a detached abdomen belonged, and there is nothing in the whole history inconsistent with the supposition that these observations really relate to a River Crab of the family Potamonidæ, of which at least one species,Pseudothelphusa dentata, is known to occur on the island of St. Vincent. As we have already seen, some of these River Crabs are quite as much land animals as the Gecarcinidæ, and they are known to have a direct development.
The Gecarcinidæ possess well-developed gills, but in addition the gill chambers are modified for air-breathing, as in some other amphibious Crabs(Ocypode,Gelasimus, etc.). Each chamber is capacious and vaulted, and the lining membrane is thick and richly supplied with bloodvessels, and is folded so as to divide off the upper part of the chamber as a sort of pocket.
The Land Hermit Crabs of the family Cœnobitidæ are found on the coasts of all tropical seas. Like the Gecarcinidæ, they visit the sea periodically for the purpose of hatching off the eggs, and the larval stages are marine. The species of the genusCœnobita(Plate XXVI.) resemble the marine Hermit Crabs in general shape, and like them use the shells of Gasteropod Molluscs as portable shelters. Where shells are scarce, other hollow objects are occasionally utilized; for example, large individuals will sometimes carry about the shell of a broken coconut, and a specimen has been seen to walk off in a cracked test-tube discarded by a naturalist who was investigating their habits. In one instance Professor Alcock saw an individual "so big that it seemed to have given up hope of finding a house, and was wandering about recklessly, with its tail behind it all unprotected."
The Cœnobites often climb into bushes in search of food, and Dr. Alcock "once found one of them busy, like a large bee, among the florets of a coconut, which made me wonder whether they may not sometimes play a part in fertilizing flowers." They are, however, by no means exclusively vegetarians.The author just quoted describes a visit to Pitti Bank in the Laccadive Archipelago, the breeding-ground of two species of terns. The ground was everywhere strewn with the dead bodies and clean-picked skeletons of the young birds. "We soon discovered that one great cause of the wholesale destruction of young birds was the voracity of swarms of large Hermit Crabs (Cœnobita), for again and again we found recently killed birds, in all the beauty of their first speckled plumage, being torn to pieces by a writhing pack of these ghastly Crustaceans. There were plenty of large Ocypode Crabs, too (O. ceratophthalmus), aiding in the carnage."
On Christmas Island Dr. Andrews found a species ofCœnobitanot unfrequently in the higher parts of the island far from the sea, and he remarks that the occurrence of large marine shells high up on the hills seemed very puzzling until it was noticed that they were brought by the Hermit Crabs.
The species ofCœnobitapossess a very curious adaptation for aerial respiration. The soft skin of the abdomen is traversed by a network of bloodvessels and acts as a kind of lung, and a pair of contractile vesicles at the base of the abdomen serve as accessory hearts in promoting a specially active circulation in that part of the body. The lining membrane of the gill chambers also appears to aid in respiration as in other terrestrial Decapods.