NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS 33 



face of the embryo (fig. 6 D, Prst) ; in the adult it is reduced to a 

 small lobe overhanging the mouth (fig. 14, Prst). Appendages of 

 the prostomium are best developed in the errant polychaetes, where 

 typically they include a pair of anterior tentacles, or "antennae" {Tl), 

 with frequently a median tentacle between them, and a pair of more 

 posterior and ventral palpi {Pip). The prostomial appendages are 

 clearly not equivalent to the parapodia of the postoral body somites, 

 but they have the same development in the larva as the parapodial 

 cirri (cf. fig. 16, A and B). Since the prostomium usually contains 

 the brain and bears the apical sense organs, it constitutes the "head" 

 •of the worm. In the absence of prostomial appendages and sense 



Fig. 14. — Head and anterior body segments of Nereis virens Sars. A, dorsal ; 

 B, ventral. 



Cirl, Cirll, tentacular cirri of first and second somites united in peristomium ; 

 E, eye ; ///, IV, third and fourth somites ; Papd, parapodium ; Perst, peristomium 

 (somites / and //) ; Pip, palpus; Prst, prostomium; Tl, prostomial tentacle. 



organs, however, the brain may be secondarily withdrawn into the 

 body, as in the earthworms (fig. 17 C, D, Br). 



The prostomium is not affected by the process of metamerism that 

 cuts the postoral body region into a series of somites. Since the 

 mesoderm bands of the larva do not proceed anterior to the mouth 

 (fig. 6F). the larval prostomium does not contain mesoderm; but 

 in later stages the mesoderm of the first somite may be extended 

 into the prostomium (fig. 10 A) and give rise to a cephalic coelom 

 and peritoneum (B, PCoel). Ordinarily the cephalic mesoderm is 

 not segmented, but according to Binard and Jeener (1928) there is 

 present in the prostomium of the spionid Scolelcpis fuliginosa a pair 

 of distinct coelomic sacs, which are continuous with the cavities of 

 the palpi, and have no connection with the coelomic sacs of the first 

 postoral somite. This fact, the authors point out, gives a new argu- 



