THE CRUSTACEA 



Each vas deferens expands in the thoracic region to form a seminal 

 vesicle (Fig. 64, v.s). The two vasa deferentia unite after entering 

 the long thread-like penis (Figs. 64, p, and 67, P) which arises at 

 the posterior end of the body on the ventral side of the anus. The 

 penis can he protruded from the opening of the shell to deposit 

 spermatozoa within the mantle-cavity of an adjacent individual, 

 but probably self-fertilisation may occur in isolated individuals. 

 The spermatozoa are filiform and motile. 



Dwarf Males. In the Operculata and in the great majority of 

 the Pedunculata all the individuals of a species are similar and 

 hermaphrodite, but in two genera of Pedunculata, Scalpellum and 



Fio. 69. 



Dwarf males ofA, Sml pell um peronii ; B, 6'. gigas ; C, S. vdutinum. Cn, vestige of mantle- 

 cavity ; E, stomach ; Gl.c, cement-gland ; S, L scutum ; T, tergum ; Te, testis ; 7s, seminal 

 vesicle. (From Gruvel's Monographi?.) 



Ibla, dwarf male individuals occur. These are attached within the 

 mantle-cavity of the large individuals, which may be either herma- 

 phrodites of the usual type, or, in some cases, purely females. In 

 the former case the males which are paired with hermaphrodite 

 individuals present a type of sexual relations not definitely known 

 to occur elsewhere in the animal kingdom except among the 

 Myzostomida, and were termed by Darwin " complemental males." 

 As regards their structure, the dwarf males show great differ- 

 ences even in closely allied species of the same genus. In one 

 group of species, including Scalpellum peranii, S. villosum, etc., the 

 male is an almost perfect miniature of the large hermaphrodite 

 to which it is attached (Fig. 69, A). The peduncle is distinct, 

 though short, and the capitular plates are well developed. The 



