OLIGOCHAETA 29I 



Other modifications are those in which the nephridia open into the 

 ahmentary canal instead of to the exterior. They may ho. peptonephridia^ 

 opening into the interior part of the ahmentary canal ; whether they 

 have a digestive function is not known. On the other hand they may 

 unite to form a longitudinal duct (or ducts) which discharges into 

 the hind end of the intestine. Whether there is any physiological 

 meaning for the variations in the segmental organs of the earthworms 

 is entirely unknown. 



There is a well-developed blood circulation. Blood flowing through 

 the parietal and dorso-intestinal vessels of each segment is collected 

 in the dorsal vessel. It is prevented from returning by an elaborate 

 system of valves (Fig. 201). Waves of peristaltic contraction beginning 

 at the hind end of the dorsal vessel and continued by the "hearts" 

 press it forwards and ventralwards into the ventral vessel which is the 

 main distributing channel. 



The aquatic Oligochaets 



As a type of these, Stylaria, belonging to the family Naididae, will 

 be shortly described (Fig. 202). This is a transparent worm rather 

 less than a centimetre long found crawling on water weed. The 

 prostomium bears minute eyes and is produced into a long filiform 

 process. In most of the segments there are two bundles of chaetae 

 on each side, the dorsal consisting of hair chaetae and needle chaetae, 

 while the ventral has only "crotchets" with a double point. The first 

 four segments have no dorsal bundles (incipient cephalization). 



The alimentary canal is simpler in character than that of Lumhricus, 

 3. gizzard being absent. The intestine is ciliated and the action of the 

 cilia brings in from the anus a current of water which probably assists 

 respiration. The testes (Fig. 199 B) develop in segment 5 and the 

 ovaries in segment 6, while a pair of spermathecae is found in the 

 testis segment. The sexual cells develop in the seminal vesicle and 

 the ovisac which are unpaired backward pouchings of septa 5/6 and 

 6/7 respectively. The male ducts open by a funnel on septa 5/6 and 

 discharge into an atrium, which is lined by the cells of the prostate. 

 While sexual individuals are often met with and can be recognized 

 at once by the appearance of the opaque clitellum in segments 5-7, 

 individuals reproducing asexually are much commoner. Chains of 

 worms attached to one another may be found, and the existence of 

 one or more zones of fission, where new segments are being formed 

 and separation of two individuals will take place, is easily observed 

 under the microscope. 



Stylaria is a delightful object of study. The operation of many of 

 the organs can be easily observed with a low power and the results 



