THE CRAYFISH 



265 



to one side of this a small, oval body. Pairing takes place 

 in September and October. The male seizes the female, 

 throws her upon her back, and passes sperm through the 

 tubular limbs of his first abdominal segment on to the 

 parts in the neighbourhood of her oviducts, the limbs of 

 his second abdominal pair aiding the process by working 

 to and fro on the hollows of the first. The sperm consists 

 of a sticky substance, secreted by the vasa deferentia, 

 carrying the spermatozoa, and 

 forms white masses on the 

 sterna of the female. The 

 eggs, which are large and 

 yolky, are laid in November. 

 The processes of the sper- 

 matozoa adhere to them, and 

 by a sudden expansion of the 

 capsule the rest of the body 

 is forced into the ovum. Each 

 egg is attached to one of the 

 hairs on the abdominal limbs 

 by a stalked shell formed of a 

 substance secreted by certain 

 glands on the sterna, and is 

 thus under the protection of 

 the mother during its de- 

 velopment. By the division 

 of the nucleus of the fertilised 

 ovum a syncitium is formed 

 which does not divide into 

 cells until a number of nuclei 

 have arisen. The young are 

 hatched at the beginning 



of the next summer. They do not differ greatly from 

 the adult, but have curved tips to the pincers, by which 

 they cling for a time to the empty shell or the abdominal 

 limbs of the mother, and are thus protected from enemies 

 and kept from being swept away by currents and so 

 eventually reaching the sea, where they would perish. 



The power of regeneration, though it is less in the 

 crayfish than in earthworms and much less than in Hydra, 

 is still considerable, A whole limb which is injured can 



Fig. 169. — The reproductive 

 organs of a male crayfish. — 

 After Huxley. 



t, Testes ; vd., vas deferens ; vd'., open- 

 ing of vas deferens on last walking 

 leg. 



