576 



COMPARATIVE ANATOMY 



CHAP. 



dorsal walls. These are the immediate anterior continuations of the 

 dorsal longitudinal musculature of the trunk. In Ptychodera, a single 

 weak layer of longitudinal muscle also develops on the ventral walls 

 of the perihsemal cavities. In Schizocardium and Glandiceps, on the 

 contrary, a transverse musculature is here developed. In the genus 

 Balanoglossus, both longitudinal and transverse muscles are wanting 

 in the ventral wall. 



Besides the muscles which have been mentioned, there are fibres 

 which traverse the perihaemal cavity transversely, chiefly in a dorso- 

 ventral direction. 



The peripharyngeal cavities are also anterior continuations of 



FIG. 460. Ptychodera minuta, diagram of the collar 

 coelom and of the anterior region of the trunk, in an 

 almost median longitudinal section (after Spengel), some- 

 what modified. 1, Anterior wall of the collar ; 2, collar 

 coelom ; 3, peripharyngeal cavity ; 4, perihsemal canal ; 

 5, buccal cavity ; 6, septum dividing the collar from the 

 trunk ; V, trunk coelom ; 8, oesophagus. 



FIG. 461. Ptychodera minuta, 

 transverse section of the body through 

 the genital region, diagram to illustrate 

 the arrangement of the coelom. d, Dor- 

 sal ; v, ventral ; 1, dorsal mesentery ; 

 2, dorsal accessory chambers of the 

 trunk coelom ; 3, lateral mesenteries ; 

 4, body epithelium ; 5, parietal wall ; 



6, visceral wall of the trunk coelom ; 



7, intestinal epithelium ; 8, intestinal 

 cavity ; 9, principal chamber of the 

 trunk coelom ; 10, ventral mesentery. 



the trunk coelom, which push in between the buccal cavity (pharynx) 

 on the one side and the collar ccelom on the other, and, in Ptychodera, 

 surround the buccal cavity. Anteriorly, they end dorsally at the 

 point where the proboscidal diverticulum of the intestine arises, and 

 laterally, at the points of attachment of the vascular folds. The 

 inner wall of the peripharyngeal cavities consists of a layer of circular 

 muscle fibres which surrounds the buccal cavity, closely applied to 

 its epithelium, and only separated from it by a limiting membrane. 



In Schizocardium, the two peripharyngeal cavities are less exten- 

 sive, they lie at the sides of the buccal cavity, without surrounding 

 it either ventrally or dorsally. Each peripharyngeal cavity forms a 



