CRA YFISH— SHRIMPS — WA TER- FLEAS 3 7 5 



there are crayfishes, inhabiting for the most part the Southern Hemisphere, which 



are much longer. It may be well to say that crayfishes are purely fresh-water, and 



that the crawfish, or rock-lobsters {Palinwrus), which differ from them in several 



ways, are only found in salt water. 



The isopod crustaceans, which live partially on land, are also 

 Water-Louse 



represented in fresh water by the common water-louse {Asellus 



uqiiaticus), inhabiting all the ponds and pools of Europe, and creeping about on 

 the bottom, or on stones and water-plants. It winters in the mud, is grey with 

 brownish yellow spots, and is under half an inch long. In deep wells and lakes 

 its place is taken by A. cavaticus, which has no eyes, is almost colourless, and 

 measures, about a quarter of an inch. 

 Freshwater One of the best known crustaceans frequenting the fresh waters 



shrimps. f central Europe is the fresh-water shrimp (Gammarus pjidex), be- 

 longing to the amphipod group of the class. It is about half an inch in length, and 

 yellowish brown in colour, with a dark brown longitudinal stripe on each side. 

 Another species of this interesting family is Rosel's fresh-water shrimp (G. fluvia- 

 tilis), distinguished by a pointed spine on the last ring of the body, and found in 

 sluggish waters in marshes and lakes. Here also dwells the well - shrimp 

 (67. puteamts), which is as common as the other two, but distinguished by the nearly 

 square leaf-like penultimate joint of the four front-legs It is found in sub- 

 terranean waters and at the bottom of lakes, and is sometimes so numerous in 

 wells that the water becomes unfit for use. 



In the so-called copepod, or oar-footed, group, the body is long and 

 Water-Flea etc ax 



distinctly divided into segments. The females of one of the common 



species, Cyclops coronatus, carry their eggs in two bladder-like bags, at the hind 



part of the body, which is pear-shaped in outline, the back being furnished with an 



appendage tipped with four long bristles. They live in shady brooks in slowly flowing 



water, swim in jerks and leaps, and are under an eighth of an inch in length. 



Another group, the ostracods, possess a bivalve shell, formed of the right and 

 left halves of the carapace, with a ligament to keep them open and a muscle to keep 

 them shut, in a way curiously suggestive of the bivalve molluscs. The best known 

 is Cypris fusca, which is found in pools and ditches in great numbers, and takes 

 refuge in the mud when they dry up, ready to appear after every shower. Yet 

 another group, the Cladocera, is so called from the branched antennae, by means 

 of which these crustaceans swim. They form an important item of food for many 

 kinds of fishes ; and are furnished with five or sis pairs of legs with leaf-shaped 

 feet. They cast their skins frequently when young, and the females, which are 

 capable of propagating without the males, carry their eggs on their back. One 

 of the commonest species is the water-flea, Daphnia pidex, which has a reddish, 

 curved, short-pointed shell. 



Of larger size, and with or without a shield-shaped bivalve shell, but always 

 with from ten to forty well-developed leaf-shaped gill-appendages, are the phyllopod 

 crustaceans. They live in ditches, pools, marshes, and similar situations, generally 

 in fresh but sometimes in salt water, although never in the sea, and are 

 characterised by the small percentage of males, which are sometimes entirely 

 wanting for several generations. In some years the stagnant pools may con- 



