ENTEROPNEUSTA HILL. 339 



The outer opening of the canal is expanded and provided with 

 thick out-turned lips. In this species the first and second gill 

 pockets have a common efferent portion into which the collar 

 canal opens (fig. 8, g.c. 1 and 2). 



Nerve Cord of Collar. The collar nerve cord is dorso-ventrally 

 flattened and band-like in shape, convex above and concave in its 

 mid-region below (fig. 7, cnc.}. As in P. sarniensis,* a continuous 

 axial canal (fig. 7, cnl.) opening both anteriorly and posteriorly 

 is present in the cellular part of the cord. The axial canal is 

 wide and dorso-ventrally compressed like the cord itself. Towards 

 its anterior end, it narrows to open to the exterior by the small 

 anterior neuropore (fig. 6, an.), the posterior neuropore is a slightly 

 larger opening. The canal is lined by a cuticular layer, and there 

 open into it numerous gland cells, especially abundant and large 

 in the ventral wall of the canal. The ventral wall is very much 

 thicker than the dorsal. As in other species of the genus, the 

 cellular part of the cord is completely invested by the fibrous 

 layer, but here the layer of fibres on the dorsal surface of the cord 

 is a very thin one. 



Dorsal Roots. As in P. australiensis, the first root may arise 

 from the collar nerve cord quite near its anterior end. The roots 

 are quite irregular, both in size, number, course, and disposition. 

 They vary in number from one to two roots, situated in the 

 anterior half of the cord, to four, seven or eight, in four specimens 

 examined. The axial canal of the cord is not prolonged into any 

 of the roots. They all possess a solid cellular core surrounded by 

 a thin fibrous layer, and are invested by the usual basement 

 membrane carrying blood. Where they join the epidermis, they 

 cause no interruption of the cells of the latter such as occurs 

 in P. minuta and in, at least, the most anterior of the roots in 

 P. australiensis. 



Trunk. Except for a thin layer of circular muscles below the 

 epidermis at the extreme posterior end of the body, forming the 

 anal sphincter, a circular muscular layer is absent below the epi- 

 dermis of the trunk, as occurs in no other described species of the 

 genus Ptychodera. 



Branchial Region. Owing to the absence of both genital pleura 

 and cushions in this region, it has in transverse section a dorso- 

 ventrally compressed ovalish outline (Plate xix., fig. 2). The 

 dorsal nerve (Plate xxi., fig. 9, dn.} lies at the bottom of a 

 deep median groove, wider below and narrow above. The 

 epidermis forming the lateral walls of the wider ventral part 

 of this groove contains numerous gland cells and stains very 

 deeply (fig. 9). Gland cells are also present in small numbers 



* Spengel Zoc. cit. 



