90 



BALANOGLOSSIDA. 



FIG. 70. Diagrams of transverse sections through the trunk of 

 Ptychodera to show the insertions of the lateral mesenteries. 

 A, behind the gill region ; B, through the hinder part of the 

 gill region (from MacBride after Spengel). 1 dorsal mesentery ; 

 2 dorsal division of coelom ; 3 lateral septum ; 4 alimentary 

 canal ; 5 ventral division of coelom ; 6 ventral mesentery. 



The muscles of the collar and trunk are entirely developed 

 from the walls of the coelom. Some of them are developed from 

 the parietal walls and some from the visceral walls both in collar 



and trunk, and 

 some from the 

 walls of the collar 

 extensions of the 

 trunk coelom. 



In the case of the 

 collar coelom the 

 whole coelomic wall 

 appears to be con- 

 verted into either 

 muscular or connec- 

 tive tissue, no por- 

 tion being left over 

 for lining epithelium 

 save in the collar 

 canals. 



The muscles of the body wall of the collar are developments of the parie- 

 tal wall of the collar coelom. They consist of an external layer of longi- 

 tudinal fibres and in the anterior region of an inner circular layer as well. 

 In the trunk the body wall muscles diminish in strength posteriorly. 

 They consist of a layer of longitudinal fibres only, except in the Ptychoderidce 

 in which there is an outer layer of circular fibres as well. The layer of 

 longitudinal fibres is interrupted in the dorsal and ventral middle lines and 

 in the submedian lines (where the branchial pores and gonads open). 



The visceral muscles of the collar are rather complicated, part of them 

 being derived from the extensions of the trunk coelomic sacs. They are 

 partly longitudinal and partly transverse. Briefly, it may be said that 

 the visceral wall of the collar coelom where it is in contact with the outer 

 wall of the peripharyngeal extensions consists of longitudinal muscular 

 fibres, whereas, where it is in contact with the gut wall, it gives rise to trans- 

 versely directed fibres. For instance, in Ptychodera all the buccal wall 

 behind the vascular fold possesses longitudinal fibres, whereas in front of the 

 vascular fold, transverse fibres only are found. In the other genera the 

 longitudinal fibres are found over the triangular area referred to above 

 (p. 89), while only the transverse fibres are found on the part in front of 

 the vascular folds. The inner wall of the peripharyngeal cavities con- 

 sists of circular fibres which, together with the partial layer of circular 

 fibres belonging to the visceral wall of the collar coelom, complete the trans- 

 verse layer of fibres round the gut in the collar region. In addition 

 to the above muscles, radial fibres are found passing across the collar 

 coelom from the visceral to the parietal wall. 



In the trunk region the coelomic space is traversed by radial muscular 

 bands and the visceral muscles. 



The dorsal walls of the perihaemal cavities give rise to longitudinal 

 muscles which almost entirely fill them. Muscles are also in some genera 

 developed in the ventral walls of these cavities. There are also fibres 

 which traverse the perihaemal cavities. 



