9 



which are used in capturing prey or in seizing foreign bodies in 

 the water. These pincer-like structures are called chelae, and the 

 whole appendage is said to be chelate. In the division to which 

 the "lake shrimp" belongs all have three pairs of chelate Legs, but 

 the members of the other divisions of shrimps have bill two pairs 

 of chelate legs. Although this is an external character, h is on 

 that account very useful, and there are other modifications in 

 struct uic accompanying these differences, so that it is very useful 

 in separating the shrimp into the groups to which they belong. 



In the lower side of the abdomen arc six pairs of appendages. 

 All of these, except the last pair, are similar in general shape as 

 well as in structure. Each appendage consists of a basal two- 

 jointed stalk, at the outer end of which are two narrow, blade 

 like processes. (Plate III.) As already mentioned, the chief 

 function of these appendages is swimming paddles, serving to 

 propel the animal slowly forwards. In the Females of many of 

 the shrimps they serve also as egg-carrying organs, and the eggs 

 are attached to them by a gelatinous secretion and are carried 

 about by the female until they hatch. The female of the " River 

 Shrimp" (Bithynis ohionis), is an excellent example of this and 

 many will be found in this condition throughout the entire sum- 

 mer. The figure of this species given at the end of this report 

 shows this habit. (Plate V, figure 1.) The last pair of abdom- 

 inal appendages are different in shape from the others and are 

 apt to be overlooked as appendages. Instead of being inserted 

 at an angle with the abdomen they lie nearly parallel to it, form- 

 ing, with the tip of the abdomen, the so-called "tail-fin." This 

 structure is used as a powerful paddle in executing rapid back- 

 wards movements when attempting to escape from a predaceons 

 form. The whole abdomen enters into the movement, which 

 causes the shrimp to move backwards in a jerky manner. 



In the present report the modifications of the first two pairs of 

 swimmerets of the males are of special interest. In the females 

 but slight modifications of these appendages are found: the 

 greatest being that the inner filaments of the first pair are very 

 much smaller than those of the following appendages and are apt 

 to be overlooked. In the males, however, the changes are very 

 pronounced. In the first pair of swimmerets in this sex the 

 inner filaments are modified into delicate, slightly wrinkled 



