CHAETOPODA 



145 



The setae vary a good deal in number and shape in differ- 

 ent species (Fig. 91), but each is the product of a single cell 

 which lies at the base of the sac from which the seta protrudes. 



The alimentary canal of many of the lower Oligochaets is 

 ciliated; in iMmbricus the lining epithelium from the mouth 

 to the gizzard secretes a cuticle, but the intestine is lined by 



Fig. 91. 



a. Penial seta of Periehaeta ceylonica. 



b. Extremity of penial seta of Acan- 



thodrUus. After Horst. 



V. Seta of Urochaeta. After Perrier. 



d. Seta of Limibricus. 



e. Seta of Criodrilus. 



modified retractile cilia. Criodrilus, which inhabits the mud, 

 and Pontodrilus, which lives on the sea-shore, have no gizzard ; 

 both these genera are also without nephridia in the anterior 

 10 or 15 segments. The typhlosole which is so characteristic 

 in the intestine of Zumhricus (Fig. 92) is also absent in the 

 latter genera as well as in Megascolides. In Bhinodrilus it 

 forms a spiral fold running round the intestine. 



The blood is contained in a series of closed vessels. The 

 plasma of the blood is usually coloured red by haemoglobin 

 which is dissolved in it, and not confined to the corpuscles. 

 Numerous flattened corpuscles float in it. The coelomic fluid 

 found in the body-cavity contains colourless amoeboid corpuscles. 



The nephridial system of leeches shows how a single pair 

 of nephridia in each somite, distinct from aU the others, may 

 arise from a scattered network. In Oligochaets a similar series 

 of stages in the developement of a single pair of nephridia in 



10 



