470 



ZOOLOGY 



peristomiwm, forms in many Polychseta a very distinct head ; the 

 prostomium in such a case bears eyes and tentacles and contains 



Fig. 308.— Sette of various Polychasta. (From Claparede.) 



the cerebral ganglion ; on the peristomium is the opening of the 

 mouth, and from it also arise the peristomial tentacles. A 



Fig 360.— Section of the setigerous sac of an Oligochsete. bj, setigerous sac ; &._>, supplementary 

 follicle with seta ; e, cleric epithelium (epidermis) ; Im, longitudinal muscles of body-wall ; 

 tn, 'in. muscles of the setigerous sac ; ?-.»i, circular muscular layer of body-wall. (From 

 Hatschek, after Vejdovsky.) 



ventral pair of prostomial tentacles, somewhat thicker than the 

 rest, are sometimes to be distinguished, and are termed the palpi. 



