I REPRODUCTIVE ORGANS 15 
able; in Maia squinado it is greatly reduced, the caeca being 
closely approximated; while in Galathea strigosa 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 supphed 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 spermatozoa 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' has observed that when the spermatozoon has 
1 Arch. f. mikr. Anat. \xvii., 1906, p. 364. 
