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 les at the base of the sac from which the seta protrudes. 
The alimentary canal of many of the lower Oligochaets is 
ciliated; in Lumbricus the lining epithelium from the mouth 
to the gizzard secretes a cuticle, but the intestine is lined by 
Fic, 91. 
a. Penial seta of Perichaeta ceylonica. 
6. Extremity of penial seta of Acan- 
AA 
‘s A x thodrilus. After Horst. 
A , 
A c. Seta of Urochaeta. After Perrier. 
ARK A 
USK d. Seta of Lumbricus. 
A\ 
ee e. Seta of Criodrilus. 
KA f 
Aah 
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 Lumbricus (Fig. 92) is also absent in the 
latter genera as well as in Megascolides. In Rhinodrilus 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 all 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 
