388 ANNELIDA. 



| 



muscular and opens to the exterior by a terminal or dorsally placed 

 anus (fig. 312). 



The vascular system is probably in communication with the body 

 cavity; it consists of a dorsal vessel, which, as in the Annelida, 

 accompanies the alimentary canal, and of a ventral vessel running 

 along the body wall. There are also branches on the alimentary canal 

 and in the tentacles. The blood is either colourless or red, and moves 

 in the same direction as in the Annelids, the current being maintained 

 both by the contraction of certain parts of the vessels and by the 

 cilia which line the walls of the vessels. The corpusculated fluid of 

 the body cavity differs from this vascular blood. 



Excretory organs. There are two sets of organs, both of which 

 may be interpreted as seginental organs. One kind, the anal vesicles 

 (fig. 314c, -46), are only present in the Chcetifera ; they have the form 

 of a pair of tufted tubes, which open, on the one hand, into the 

 body cavity by numerous ciliated funnels and, on the other, into the 

 rectum. The other kind, known as the brown tubes (fig. 312, BcV) 

 or ventral glands, are placed (one or more pairs) in the anterior part 

 of the body ; they also open into the body cavity by a ciliated funnel, 

 and to the exterior on the ventral surface. The latter, like the seg- 

 mental organs of Annelids, assume the function of seminal vesicles 

 and of oviducts. 



Generative organs. The Gephyrea are of separate sexes. There 

 are, however, remarkable variations both in the generative gland ^ 

 and their ducts. In Phascolosoma amongst the Achceta (accordin & 

 to Theel) the generative glands lie at the root of the ventral retractor 

 muscles of the proboscis, and form a ridge from which the generative 

 products are set free. Spermatozoa or ova in various stages of 

 development are found in the body cavity, and thence are carried to 

 the exterior through the two brown tubes (segmental organs) which 

 open on the ventral side. 



In Bonellia among the Chcetifera the ovary, which has the form of 

 a thin cord (fold of the body wall) in the posterior half of the body, 

 is attached by a short mesentery to the nerve cord. From the ovary 

 the oya fall into the body cavity, and thence pass into the neigh- 

 bouring single uterus (fig. 314, b, U), which is provided at its base 

 with a trumpet-shaped opening (Tr) and opens to the exterior on 

 the ventral surface behind the mouth. This uterus ought probably 

 to be considered morphologically as a segmental organ, which has 

 only been developed on one side. The generative organs of the 

 small Turbellarian-like males which are met with in the uterus of 



