212 



COLLEGE ZOOLOGY 



Protopodite 



Muscle 



Breaking plane 



Figure 119. Left, different views of crayfish sperms, greatly magnified. Right, posterior view 

 of basal region of second walking leg of the crayfish, showing the line where the break occurs 

 in autotomy, and the special muscle concerned. Arrow indicates the direction of pull of the 

 muscle involved in the reflex. 



maturation divisions and then develop into 

 sperms. These are flattened spheroidal bodies 

 when enclosed within the testis or vas 

 deferens; but, if examined in water or some 

 other liquid, they are seen to uncoil, finally 

 becoming star-shaped (Fig. 119). 



The sperms remain in the testes and vasa 

 deferentia until copulation takes place. As 

 many as two million sperms are contained 

 in the vasa deferentia of a single specimen. 



Female reproductive organs. The two 

 ovaries resemble the testes in form and are 

 similarly located in the body (Fig. 116). A 

 short oviduct leads from near the center of 

 the side of each ovary to the external open- 

 ing in the base of the third walking leg. 



Oogenesis. The primitive germ cells in the 

 walls of the ovary grow in size and become 

 surrounded by a layer of small cells, the 

 follicle, which eventually breaks down, al- 

 lowing the eggs to escape into the central 

 cavity of the ovary. At the time of laying, 

 the ova pass out through the oviduct. 



Fertilization and development. The 

 sperms are transferred from the male to the 

 seminal receptacle of the female during 

 copulation, which usually takes place be- 

 tween early spring and autumn. The seminal 

 receptacle is a cavity in a fold of cuticle be- 

 tween the fourth and fifth pairs of walking 

 legs. The female lays her eggs several weeks 

 to several months after copulation; the ova 



are fertilized by the sperms when laid. The 

 eggs are fastened to the swimmerets with a 

 sticky substance and are aerated by being 

 moved back and forth through the water. 



The cleavage of the egg is superficial, and 

 the embryo appears first as a thickening on 

 one side. The eggs hatch in 2 to 20 weeks, 

 and the larvae cling to the egg stalk. In 

 about 2 to 7 days they shed their euticular 

 covering, a process known as molting. Cast- 

 ing off the covering of the body is not pe- 

 culiar to the young, but occurs in adult cray- 

 fishes as well as in young and adults of 

 many other arthropods. In the larval cray- 

 fish the cuticle of the first stage becomes 

 loosened and drops off. In the meantime, 

 the epidermal cells have secreted a new 

 covering. Molting is necessary before growth 

 can proceed since the exoskeleton is hard 

 and nonelastic. In adults it also is a means 

 of getting rid of an old worn-out coat and 

 acquiring a new one. Over a period of some 

 time before the molt, a quantity of calcium 

 from the old exoskeleton is absorbed and 

 distributed by the blood, especially to the 

 stomach, where it is deposited in calcareous 

 bodies, the gastroliths. The formation of 

 gastroliths is under the control of an endo- 

 crine gland. The young stay with the mother, 

 attached to her swimmerets with their 

 chelae (see headpiece at beginning of this 

 chapter) until they can shift for themselves. 



