The Arthropoda 



325 



them very thoroughly. When the eggs are laid, they adhere to the swim- 

 merettes which are moved back and forward through the water, thus 

 aerating the eggs. It takes from five to eight weeks for the eggs to 



Fig. 209. 



A. Male reproductive organs of crayfish, t., Testes; vd., vas deferens on last 

 walking leg. (After Huxley.) 



B. Female reproductive organs of crayfish, ov., Ovaries; ov'., fused posterior 

 part; od., oviduct; vu., female aperture on the second walking leg. (After Suckow.) 



C. Spermatozoa of a crayfish. C. Whole spermatozoon from above; D, part, 

 enlarged, from the side, cps., Capsule; pr., stiff processes. (After Borradaile.) 



hatch, the larvae clinging to the egg shell. In about" two days the first 

 molting or ecdysis takes place. Like any animal possessing an exoskele- 

 ton the crayfish finds it impossible to grow without splitting its exterior 

 covering and getting a new one to take its place. 



The young stay with the mother about a month, then shift for them- 

 selves. Crayfish attain an age of approximately three or four years. 

 They molt at least seven times during the first summer. 



REGENERATION 



We have seen how the earthworm, if it is divided in a region pos- 

 terior to the vital organs, will grow a new tail for the forepart, as well 

 as a new tail-like portion on the tail itself. In the latter case, the animal 

 starves to death, because there is no way of eating. 



With the flatworm Planaria, all manner of fantastic forms may be 

 grown by cutting off, or splitting, or grafting. The crayfish, too, pos- 

 sesses the power of regeneration to some extent, though nowhere nearly 

 as much as the worms. If a leg, eye, or pincer is destroyed (Fig. 210), 

 the animal grows a new appendage, though in place of an eye, it may, 

 and often does, grow an appendage quite similar to one of the walking 

 feet, or even a pincer, depending on how much of the original appendage 

 was destroyed. 



AUTOTOMY 



An interesting condition of the crayfish, as well as of some of the 

 other crustaceans, is the breaking off, by the animal itself, of one or more 

 of its legs when caught in a position where it seems incapable of extri- 

 cating itself. 



