OLIGOCHAETA 



293 



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. 



Fig. 203. Blood circulation in Lumhriculus variegatus. After Haffner. A, 

 Head and anterior region showing dorsal and ventral vessels joined by a 

 network of vessels round the gut. B, Single segment of the middle region 

 with a much closer plexus. C, Posterior end with a continuous sinus round 

 the gut connected at intervals with the dorsal and ventral vessels. An. anus ; 

 bl. blind contractile sac of the dorsal vessel {d.v.) ; M. mouth ; pi. plexus ; 

 sin. sinus; v.v. ventral vessel. 



Two common genera, Tubifex and Lumhriculus, 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. 



It lives in the mud at the bottom of ponds and lakes with its head 

 buried and its tail waving in the water; the latter movements are 

 respiratory. They draw water from upper layers which contain more 



