The Ostracoda have bivalve shells that cover the whole animal and the 

 legs are cylindrical rather than flattened. When inactive they look like tiny 

 clams, hut in action the shell opens and the appendages flash out and perform 

 their task of propulsion most efliciently. Like the Cladocera, the Ostracoda 

 are abundant in most bodies of fresh water and play an important part as food 

 for young fishes. 



The fresh-water Copepoda are mostly free-living, but a few are serious 

 parasites upon fish. One of these parasitic forms, Argtthis, is commonly called 

 the carp louse, but it is also common upon goldfish and some others. Its body 

 is much modified and resembles a fish scale. It can swim well, but spends 

 much of its time crawling upon or hanging in the gill chamber of fish, sucking 

 the blood of its host. The other forms of parasitic Copepoda are, in the female, 

 even more modified in body form and are incapable of swimming, as their legs 

 are rudimentary. The mouth appendages are modified into sucking or clasp- 

 ing organs. Their bodies are worm-like or sac-like. Two long sacs of eggs often 

 hang from the rear. The free-living Copepoda are well represented by the 

 common genus Cyclops, so named because of the single red eye in the center 

 of its head. It has a pear-shaped body ending in a long tail, and a pair of large 

 egg sacs are usually towed by the female. 



The Entomostraca are easily collected by means of a plankton net. Many 

 of them will thrive and multiply in an aquarium, provided no small fish or 

 hydra are present to devour them. Some magnification is necessary in order 

 that their distinctive characters may be determined. If the animal is too active 

 in a water mount under the microscope, a weak solution of glycerine may be 

 used as a mounting medium to retard motion. The collector is often puzzled 

 by a great variety of forms, which on close study may turn out to be immature 

 stages of a few common forms. A young phyllopod or copepod usually hatches 

 from the egg as a flat, oval body with three pairs of appendages, a state called 

 the nauplius stage. As it grows a series of moults occurs, each one bringing it 

 nearer to the adult condition in regard to the number of segments and develop' 

 ment of appendages. Most of the parasitic copepods omit the earlier stages, 

 hatching in forms much resembling Cyclops and then going through a process 

 of degeneration. 



The Malacostraca or soft-shelled animals were so called by Aristotle, who 

 used the term to separate them from the Mollusca or hard-shelled animals. Three 

 groups are common in fresh water, and a fourth is represented by one fresh' 

 water species, Mysis relicta, which is found in the Great Lakes. The name 

 ''relicta" was given to it because it was assumed to be a "marine relic", which 

 implied that the great lakes of Europe and America in which it occurs had once 

 been inland seas or inlets from the ocean. The other groups are the Decapoda, 

 including the crayfish, shrimps and prawns; the Amphipoda or scuds; and the 

 Isopoda. 



174 



