TRUNK COELOM. 89 



ventral vessel of the trunk with the efferent vessels of the heart. 

 They are obviously to be compared with the glomerulus of the 

 proboscis region. 



The trunk coelom extends throughout the whole of the body 

 behind the collar. It is a more continuous cavity than that of 

 the collar and is lined throughout by an epithelium and does not 

 open * to the exterior. The dorsal and ventral mesenteries are 

 persistent over the greater part of their course. 



The coelom of the trunk, though completely separated from 

 that of the collar, sends forwards diverticula into the collar 

 region. There are two pairs of these extensions. Dorsally on 

 each side of the dorsal blood vessel of the collar, between the 

 collar nerve-cord and the gut, there is a tubular extension of the 

 trunk coelom which extends throughout the whole collar region 

 as far as the insertion of the neck of the proboscis ; these are the 

 parihaemal cavities (Fig. 69, j). They are largely occupied by 

 longitudinal muscular tissue which is almost entirely developed 

 from their dorsal walls. The other pair of collar extensions of 

 the trunk coelom constitute the peripharyngeal cavities (Fig. 

 69, 8). These are not present in all genera (see above, p. 87). 

 They lie between the ventral wall of the collar coelom and the 

 gut. In the Ptychoderidse they entirely surround the gut except 

 in the dorsal and middle ventral lines as far forwards as the 

 transversely directed vascular folds described on p. 88. In 

 the other genera in which they are present, they also extend as 

 far forwards as the vascular folds, but these being obliquely 

 directed, they occupy a triangular area on each side of the 

 pharynx. 



In the Ptychoderidce the anterior part of the trunk coelom is divided on 

 each side into a dorsal and ventral portion by a septum called the lateral 

 septum or accessory mesentery (Fig. 70). The lateral septum passes in 

 the hinder part of its extent from the body-wall, where it is inserted along 

 the submedian line, to the gut wall ; while anteriorly it shifts its gut- wall 

 attachment to the body wall near the dorsal mesentery (Fig. 70, B). 

 Moreover, the dorsal section of the coelom gradually dwindles anteriorly 

 and ceases altogether in the branchial region. Posteriorly the lateral 

 septum ends freely and the dorsal cavity opens into the posterior part of 

 the coelom. 



* Willey has described structures in Spengelia, in relation with the first 

 gill-pouch and the perihaemal cavities, which he regards as vestiges of a 

 pair of coelomic pores for the trunk coelom. 



