14 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8 1 



(Pst), though the peristomium (Prst) is sometimes called a part of 

 the " head ". The prostomium varies in size from a small lobe over- 

 hanging the mouth, to a large area extended posteriorly into the 

 dorsal region of the peristomium (B). It bears the principal sensory 

 organs, eyes and tentacles (O, TI), of the anterior part of the worm. 

 The alimentary canal extends entirely through the body from the 

 mouth at the anterior ventral edge of the peristomium to the anus 

 at the end of the periproct. 



The nervous system of the adult annelid consists of a brain located 

 before or above the oesophagus, derived from the ectodermal apical 

 plate of the prostomium of the trochophore (fig. 7 F, AP), and of 

 a ventral nerve cord of segmental ganglia and intervening connec- 

 tives, formed from two nerve strands developed from the ectoderm 



Fig. 10. — Anterior end of an adult polychaete annelid worm, Nereis virens. 



A, lateral view. B, dorsal view. 



Mth. mouth; 0, ocellus; Pip, prostomial palpus; Ppd, parapodium ; Prst. 

 peristomium ; Pst, prostomium ; Tl, prostomial tentacles. 



of the ventral body wall and prolonged from the ventro-lateral nerve 

 strands of the trochophore. The ventral cord, therefore, is connected 

 with the brain by a pair of connective nerve cords passing to the sides 

 of the oesophagus. Usually, the two ventral nerve strands are united 

 along the midline in adult worms, but in certain polychaete forms 

 (Serpulidae) the two cords are said to remain separate, though con- 

 nected by transverse, interganglionic cominissures in each segment. 



In the Arthropoda, body segmentation begins during an early em- 

 bryonic stage, and the somites are added, in general, as in the annelids, 

 from before backward by the differentiation of new segments behind 

 the last one formed. In some of the arthropod groups, segmentation 

 is completed in a postembryonic stage; in insects (except Protura), 

 however, the somites are all defined before the creature leaves the 

 Qgg, and the typical sequence of segmentation is not always followed. 



