Part 9 (1/2)
[Illustration: FIG 58--_Bythotrephes longimanus_, FEMALE, WITH EMBRYOS IN THE BROOD-SAC 12 (After Lilljeborg)]
_Leptodora_ is further remarkable on account of its s, 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 mass of a jelly-like substance which it secretes A similar envelope of jelly is found in soh 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 alenus _Canthocamptus_ comprises species of s only a single egg-packet The plankton of lakes and ponds includes species of _Diapto antennules The latter are held out stiffly while the animal swi occasional sudden leaps by -packet is single The develop females of _Cyclops_ in a jar of water, when the nauplius larvae will soon hatch out
[Illustration: FIG 59--_Diaptoh the Copepoda, unlike the Cladocera, are not parthenogenetic, it has been found that certain species of _Diapto or drying In the early part of the breeding season the eggs have thin shells, and they hatch after a short tis are produced, which lie dor It has recently been discovered that species of _Cyclops_ and _Canthocae, in which the animal surrounds itself with a cocoon-like capsule of lands on the surface of the body and limbs The encapsuled ani 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 sas of other species
Nuenus _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 Weis a colony of _Cypris_ was kept in an aquariu the whole of that ti exclusively by parthenogenesis Probably in all species the eggs survive drying
The common ”Freshwater Shrimp” (_Gammarus pulex_), which has already been described, e number of Amphipoda, for the most part closely allied, which are widely distributed in ions of the world, with the exception of the tropics _G pulex_ itself ranges fros 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 ay _Gammarus_ and its allies contrive to spread from one locality to another
The little fresh-water Isopod _Asellus aquaticus_ (Fig 60) is conized by its general rese antennae, and with a pair of long, slender, forked uropods projecting behind The species is widely distributed in Europe, and other species of the saenera are found in North America
[Illustration: 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, for 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 froard to the mode of distribution of the fresh-water Isopoda, there is the sas are carried in a brood-pouch, and do not seeht It is no doubt in consequence of this that the fresh-water species and genera of both Ah 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 alreadyno free-swie Fros are carried by the female, the Crayfish cannot be transported froencies 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 lish species is stated to leave the water occasionally, and to n countries are still more truly amphibious in their habits It is clear, however, that the means of dispersal of the Crayfishes are very lieographical 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
[Illustration: 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 He 61), but in each case they are practically confined to the te tropical zone The Northern Crayfishes, foruished, aes on the first abdominal somite, at least in the es on that soarded as constituting a distinct faeneral 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 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 japan These far eastern Crayfishes, however, differ so much from the typical species of _Astacus_ that they are now placed in a subgenus (soenus), _Caenus _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, nuenus, _Caes froenus is allied in certain respects to the _Cambaroides_ of Eastern Asia If the systeenera have been properly interpreted, it is by no means easy to understand in ay their present distribution has been brought about
[Illustration: _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 enus _Paranephrops_ occurs, in Australia and Tasaeus_ (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 noidely 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 hether 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 roups of ani 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 fro 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 chi operations by interfering with the harvesting aeus_ (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