190 COMPARATIVE ANATOMY. 



segments^ as in the Scoleina. The generative products formed on 

 the wall of the body break off when they become mature ; or they 

 become free whilst still unripe, and float in the coelom (Fig. 93), in 

 which they further develop. The looped canals here also serve 

 as efferent ducts for both male and female generative matters ; but 

 this matter needs more careful investigation. Even in the Gephyrea, 

 as has been remarked above (§ 144), the organs which in them are 

 the homologues of the looped canals aid in the generative function, 

 and present still more remarkable modifications ; which require to 

 be more closely examined. 



A special place must be assigned to the generative organs of the 

 Rotatoria. They have nothing in common with those of Chajtopoda 

 save their dioecious character, and they are distinguished from 

 them, as from all Anuulata, by the fact that they are not meta- 

 merically repeated. The sexes differ not only in the characters of 

 their reproductive organs, but also by the rest of their organisation. 

 The males are not only smaller in size, but several of their systems of 

 organs are atrophied, especially the enteric canal. The testis consists 

 of a single tube, which opens at the hinder end of the body ; accessory 

 glandular tubes are sometimes connected with it. In the female, the 

 flattened ovary is ventral in position, and opens into the cloaca by 

 means of a short oviduct. Parts of the duct are widened and serve 

 as receptacles for the eggs ; it thi\s forms a uterus, and in some 

 species the eggs are developed into embryos before they leave it. 



§ 156. 



The generative products have the same form in most 

 divisions of the Vermes. The egg is represented by a more or less 

 modified cell. In the Nemathelminthes, the ova are formed in a 

 peculiar manner; they are budded off from a common nucleated 

 chord of protoplasm, which forms the contents of the tubular ovary. 

 A large number of eggs are produced at one time, while the rest of 

 the protoplasm forms the axis of the chord (the rhachis), and is 

 surrounded by wedge-shaped ovarian buds. The same thing 

 happens in the Hirudinea ; the ovaries of HasmojDis contain a 

 coiled filament, which corresponds to this rhachis ; and on this 

 the ovarian germs are budded. The ova are connected with the 

 filament by a thin envelope, which is drawn out into a stalk. 

 In Nephelis there is no chord, and the ovarian germs form groups 

 of cells. In all forms which have a yelk-gland it is not the egg- cell 

 only which forms the material for the development of the embryo, 

 but this is supplemented by the products of the yelk-gland — yelk- 

 cells (cf. § 147). The structure, therefore, which appears to be the 

 egg consists in them of a complex of cells, one only of which has 

 retained the value of an egg-cell. In almost all cases they are 

 surrounded by an investment, which varies greatly in character. In 

 some it is a layer of albumen only, in others this layer is sm'rounded 

 by a hard shell. 



