OLIGOCHAETA 259 



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 oiLumbricus, 

 a 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. 188 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 

 w^orms 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 

 form a useful supplement to work with Lumbricus in understanding 

 oligochaet organization. 



From the above account it will be seen that Stylaria differs from 

 Lumbricus not only in its small size and transparency but also in the 

 number and appearance of the chaetae — which give it a certain 

 resemblance to the Polychaeta. The reproductive organs, however, 

 are entirely different from those of the latter group and it is in this 

 system that the real contrast between polychaet and oligochaet lies. 



The aquatic oligochaets when they are of small size often show re- 

 duction of the vascular system, ciliation of the under surface (in one 

 form, Aeolosoma), and a nervous system of embryonic type. These are 

 characters which may be primitive but, as in the archiannelids, so 

 here, they are probably the results of simplification ; it is generally 

 agreed that the replacement of sexual by asexual reproduction is a 

 secondary feature, and the frequency with which it is found in the 

 aquatic Oligochaeta shows them to be, on the whole, specialized types. 



Two common genera, Tubifex and Lumbriculus, are larger worms 

 which in their appearance have more resemblance to earthworms. A 

 brief description of them follows. 



Tubifex. A small red worm with rather numerous chaetae in the 

 dorsal and ventral bundles belonging to various types; without 

 gizzard; testes and ovaries in segments 10 and 11 respectively. 



17-2 



