282 ZOOLOGY. 



This suborder presents a beautiful series of increasingly 

 complex forms, as we ascend from Limnetis to BrancJiipus. 

 In Limnetis the bivalve shell encloses the ani- 

 mal, and is the size of a small flattened pea. 

 There are from ter to twelve feet - bearing 

 segments. L. Gouldii Baird (Fig. 238) is very 

 rare in Canada and New England. The shell 

 of Limnadia is thin, oval, and there are from 

 eighteen to twenty-six feet-bearing segments. 

 L. (Eulimnadia) Agassizii Packard (Fig. 239) 

 inhabits small pools in Southern New En- 

 gland. The shell of Estheria (Fig. 241, Es- 

 240 -w re th er i a Belfragei Packard) is sometimes mis- 

 leg of male Esthe- taken for that of the fresh -water mollusks 

 mis. a, hand; b, Cyclos and Pisidium. The males of the fore- 

 30dy ' going genera have the first pair of feet modi- 

 fied to form large claspers (Fig. 240). 



In Apus the abdomen projects beyond the large carapace, 

 and ends in two long many-jointed appendages. There are 

 about sixty pairs of feet, each foot 

 divided into several leaf -like lobes, 

 wherein respiration is carried on. 



These Phyllopods usually swim upon 

 their backs, as in the species of Bran- 

 chipus. The females chiefly differ 

 from the males in the presence of an ^ _ ghell of mheria 



Orbicular egg-Sac On the eleventh pair Sel/ragei, enlarged three 



of feet, the sac being a modification of 

 two of the lobes of the feet, and containing but a few eggs. 

 Apus cequalis Packard (Fig. 242, Fig. 244 A, represents the 

 larva of a European Apus) inhabits pools in the western 

 plains. Lepidurus differs from Apus in having the telson 

 spoon-shaped instead of square. L. Couesii Packard (Fig. 

 243) occurs on the Rocky Mountain plateau in Utah and 

 Montana. It is an interesting fact in zoo-geography that 

 there are no species of Apus and Lepidurus east of the west- 

 ern plains. Apus has been found by Siebold to reproduce 

 parthenogenetically. 



The various species of Branchipus and Artemia have no 



