ix EXTEROPNEUSTACCELOMIC SACS 577 



triangle, whose sides are constituted as follows. The first (posterior) 

 side corresponds with its origin out of the trunk ccelom ; the second 

 (dorsal) with the lateral edge of the perihaemal cavity; the third 

 (anterior and lower) with the line of insertion of the vascular fold. 

 There is a corresponding limitation and circumscription of the trans- 

 verse muscle fibres on the lateral walls of the buccal cavity. Never- 

 theless a closed muscular envelope is formed: (1) dorsally, by means 

 of the transverse musculature on the lower walls of the perihaemal 

 cavities ; (2) ventrally, by the transverse musculature of the collar 

 coelom. 



Peripharyngeal cavities are found, not only in Ptychodera and Schizocardium, 

 but in Balanoglossui Kowalevskii, in the same form as in Schizocardium, but pro- 

 vided with longitudinal instead of transverse muscles, not belonging, however, to 

 the inner or visceral wall, but to the outer parietal wall which is in contact with 

 the collar ccelom. 



In Ptychodera, a further complication occurs in the divisions of the trunk 

 ccelom. In the anterior hepatic and the branchiogenital regions, an accessory or 

 lateral mesentery (Fig. 461) occurs on each side dorsally in the submedian line, in 

 addition to the two principal (median) mesenteries. This accessory mesentery runs 

 from the intestine to the integument, dividing the ccelom at this point into four 

 chambers, two large, ventral, principal chambers, and two small dorsal accessory 

 chambers. The accessory chambers open posteriorly into the principal chambers, 

 the accessory mesentery disappearing ; anteriorly, they narrow and end in the 

 branchial region. They are here no longer in contact with the intestine, but only 

 with the integument, the accessory mesenteries here shifting their visceral edges 

 of attachment on to the integument, in a manner which cannot here be described 

 more in detail. 



By far the greater part of the walls of the trunk coelom are 

 taken up in the formation of musculature. The parietal wall most 

 especially becomes differentiated into a powerful dermo-museular tube 

 which gradually diminishes in strength posteriorly. 



The most important and constant part of this dermo-muscular 

 tube is the longitudinal musculature. 



The longitudinal musculature, which is specially strongly developed on the 

 ventral side of the body, in the genital folds (where these are developed), and on 

 the dorsal side in the branchial region, is interrupted in the dorsal and ventral 

 median lines by the median mesenteries. A similar interruption takes place in 

 the branchiogenital region in the submedian lines, in which the gonads, and, in the 

 genera Balanoglossus, Gflandiceps, and Schizocardium the gills also, open. 



By these four lines of interruption, the longitudinal musculature is divided 

 into two dorsal and two ventrolateral areas. (B. canadensis has two streaks on 

 each side free from muscle, and gonads open in both.) 



Each longitudinal fibre runs in a curve between two points, one behind the 

 other, on the limiting membrane of the body epithelium. Each fibre thus crosses 

 numberless others. 



In Ptychodera, in addition to these, an outer circular muscle 

 layer is also differentiated from the parietal wall of the trunk coelom ; 

 the fibres of this layer pass through the mesenteries. 



VOL. II 2 P 



