I REPRODUCTIVE ORGANS I 5 



able ; in Main squinado it is greatly reduced, the caeca being 

 closely approximated ; while in Galathea striyosa the caeca are 

 greatly reduced, and the mid-gut as a separate entity has almost 

 disappeared. The relation of these variations to the habits of 

 the different crabs and to their modes of development is un- 

 known. 



The reproductive organs usually make their appearance as 

 a small paired group of mesodermal cells in the thorax compara- 

 tively late in life ; and neither in their early development nor 

 in the adult condition do they show any clear signs of segmenta- 

 tion or any connexion with a coelomic cavity. The sexes are 

 usually separate, but hermaphroditism occurs sporadically in 

 many forms, and as a normal condition in some parasitic groups 

 (see pp. 105-107). The adult gonads are generally simple paired 

 tubes, from the walls of which the germ-cells are produced, and 

 as these grow and come to maturity they fill up the cavities of 

 the tubes ; special nutrient cells are rarely differentiated, though 

 in some cases (e.g. Cladocera) a few ova nourish themselves by 

 devouring their sister-cells (see p. 44). The oviducts and vasa 

 deferentia are formed as simple outgrowths from the gonadial 

 tubes, which acquire an opening to the exterior ; they are usually 

 poorly supplied with accessory glands, the epithelium of the 

 canals often supplying albuminous secretions for cementing the 

 eggs together, while the lining of the vasa deferentia may be 

 instrumental in the formation of spermatophores for transferring 

 large packets of spermatozoa to the female. In the vast 

 majority of Crustacea copulation takes place, the male passing 

 spermatophores or free spermatozoa into special receptacles 

 (spermathecae), or into the oviducts of the female. The sperma- 

 tophores are hollow chitinous structures in which the sperma- 

 tozoa are packed ; they are often very large and assume charac- 

 teristic shapes, especially in the Decapoda. 



The spermato/oa show a great variety of structure, but they 

 conform to two chief types the filiform, which are provided 

 with a long whip-like flagellum ; and the amoeboid, which are 

 furnished with radiating pseudopodia, and are much slower in 

 their movements. The amoeboid spermatozoa of some of the 

 Decapoda contain in the cell-body a peculiar chitinous capsule, 

 and Koltzoff 1 has observed that when the spermatozoon has 

 1 Arch.f. ,aikr. Anat. Ixvii., 1906, p. 364. 



