INVERTEBRATA. 



321 



Fig. 163. REPRODUCTIVE ORGANS OF 

 FEMALE CRAYFISH. 



(After Huxley.) 



having no moving flagellum. Instead there is a radiating 

 series of stiff spokes (fig. 164). The ova are large (about 



g inch diameter), containing 

 much yolk : they are red in 

 colour when ripe, but vary 

 from white and yellow to 

 purple during development; 

 so that the ovary is usually 

 a striking object in dissection. 

 The seminal fluid, largely 

 secreted by the walls of the 

 vas deferens, is white and 

 sticky, and is smeared by 

 the male's first abdominal appendages over the abdominal 

 sterna of the female. When the ova are laid they are 

 attached to the swimmerets, and now fertilization is effected. 

 Segmentation is centrolecithal, i.e. the yolk forms a 

 central mass, around which the nuclei derived by division of 

 the first segmentation-nucleus are arranged. Hypoblast 

 is formed by an invagination on what will be the ventral 

 surface, and on this surface the main part of the embryo 

 develops (contrast the dorsal 

 development in the chick). 

 There is no free larval stage, 

 although in most marine 

 Crustacea such stages occur. 

 Details of the development, 

 without comparison with 

 other Crustacea, would be of 

 little interest. The young 

 when hatched are very like 

 the parent : they cling to the 

 mother's swimmerets persist- 

 ently, until their first moult, 



after which they do SO only Fig> 164. SPERMATOZOON ov CRAYFISH. 



when alarmed. During the 



first year, when growth is rapid, moulting recurs two or 



three times. 



26. Histology. 



ZOOL. 



Two correlated points in the histology 



21 



