154 GENERAL ZOOLOGY 



the muscle bundles are attached to the inner surface of the 

 exoskeleton. This is just the opposite of the relations of 

 muscles to skeleton in our own bodies. 



The Reproductive System. The three-lobed organ which 

 produces the germ cells lies just beneath the heart. In the 

 male this is a spermary, or testis, while in the female it 

 is called an ovary. In both sexes a pair of ducts lead from 

 the reproductive organ (or gonad) to a pair of openings on 

 the bases of certain walking legs. These sperm ducts of the 

 male open on the basal segment of either fourth walking 

 leg. In the female the oviducts lead to similar openings on 

 the second walking legs. 



Development. Cambarus limosus lays its eggs in the spring. 

 Most species of crayfishes lay their eggs then. The princi- 

 pal burrowing species, Cambarus diog'enes, lays its eggs in 

 April and they hatch in May, while a river species, Cambarus 

 immu'nis, lays its eggs in the fall and they hatch in the fol- 

 lowing spring. When the egg-laying season arrives the male 

 deposits spermatozoa in a shallow cup called the annulus on 

 the ventral surface of the female between the fourth pair 

 of legs. The mass of spermatozoa stays in the annulus till 

 the female lays her eggs. It is not certain how long the 

 spermatozoa lie there before the eggs are laid. However, 

 when the eggs are discharged from the oviducts they pass 

 back over the mass of spermatozoa. Fertilization is accom- 

 plished when a spermatozoon enters an egg. The fertilized 

 eggs are fastened to the swimmerets by a glutinous sub- 

 stance. There the embryos develop, and the young, when 

 they hatch, remain clinging to the female's swimmerets by 

 their chelipeds. The young crayfishes are not set free until 

 they are able to care for themselves. 



Relation to Environment. Crayfishes live in a great variety 

 of places. They are fresh-water animals. As a rule they 

 crawl on the bottom of rivers, brooks, and ponds, conceal- 



