584 



EVOLUTION OF ANIMALS 



Part V 



rest of the skeleton and crayfishes cannot keep their balance until they have 

 another supply of sand grains. An experiment made upon shrimps is easily 

 repeated on crayfishes. Newly molted ones are placed in an aquarium of 

 filtered water, clear except for a scattering of iron filings dropped into it. After 

 exploring the bottom a while the crayfishes pick up the filings with their pincers 

 and place them in the statocysts. If an electromagnet is then moved about in 

 the water the crayfishes will follow it. According to the position of the mag- 

 net, they roll from side to side or lie on their backs and stab the air with their 

 legs. The exercise might be the preview of a human dance. 



The two compound eyes are on stalks, movable independently, one to the 

 right and one to the left or otherwise. Each is composed of some 2500 simple 

 eyes or ommatidia. Seeing through one or another of these is like seeing 

 through a telescope pointed at a starry sky; through one there is a star; through 

 another there is darkness. Many simple eyes together bring a picture put 



-TESTIS 



OVARY 



OPENINGS OF THE OVIDUCTS 



SPERM DUCT 



5TH WALKING 

 LEG 



OPENINGS OF THE SPERM DUCTS 



Fig. 29.13. A, female reproductive system of the crayfish. B, male reproductive 

 system of the crayfish. (Courtesy, MacDougall and Hegner: Biology. New York, 

 McGraw-Hill Book Co., 1943.) 



together like a dissected puzzle as shown by photographs which have been 

 taken through parts of the eyes of insects. With such movable eyes as those 

 of crayfishes the pictures must be different in each one. Sight is essentially the 

 same in crustaceans as in insects. 



Reproduction and Life History. Crayfishes mate in September and through 

 November of their first year. At that time, sperm are passed along the spe- 

 cialized appendages of the male to the seminal receptacle of the female, a 

 cavity in a fold of cuticle on the mid-ventral line between the fourth and fifth 

 pairs of legs (Fig. 29.13). The eggs are laid in April while the females are 

 still within the burrows. Before spawning she cleans the underside of her 

 abdomen, picking it over meticulously with her pincers. Then she lies on her 

 back, with her abdomen curved so that it makes a bowl. Presently a gluey 

 secretion flows out from the cement glands and over the bases of the swim- 



