234 



ENTEROPNEUSTA FROM THE SOUTH PACIFIC, 



COLLAR. 



Collar Nerve-Cord and Roots. 



The dorsal nerve-cord in the collar of Pt. flava is a true medullary tube, possessing 

 a continuous central canal which communicates with the exterior, in front and behind, 

 by the anterior and posterior neuropores. In Pt. minuta and in many other species, 

 the lumen of the collar nerve-cord is broken up into a large number of separate 

 medullary cavities. The possession of a continuous axial canal is particularly character- 

 istic of the subgenus Chlamydothorax, and I regard it as most certainly representing 

 the more primitive condition. Such a continuous central or axial canal in the medullary 

 tube of the collar-region, occurs exclusively among the Ptychoderidae, namely, in all 

 the species of the subgenus Chlamydothorax, and in certain other isolated cases, 

 e.g. Pt. sa.rniensis, Pt. hedleyi, Pt. aperta and Pt. ruficollis n. sp. 



The collar nerve-cord is united at intervals with the epidermis by means of un- 

 paired dorsal roots in all Ptychoderidae. These roots are either solid or contain a lumen 

 communicating with the central canal. 



I attach special significance to these roots, and will therefore give a serial account 

 of my observations. 



Series i.' The first root arises shortly behind the orifice of the stomochord as 

 a hollow diverticulum from the medullary tube to the right of the median line, and 

 runs for some distance horizontally backwards, so that in transverse sections through 

 the entire animal the root is also cut transversely to its long axis. 



The second root is also hollow, and arises nearly in the median line in front of 

 the point at which the first root meets the epidermis. It is also directed backwards, 

 accompanying the anterior free edge of the dorsal septum of the collar. The first root 

 maintains its calibre until it passes into the epidermis, while the second root becomes 

 somewhat attenuated towards its distal end, although it retains its minute central 

 lumen to the end. 



The third root is obviously vestigial in its nature. It appears as a solid bud from 

 the dorsal wall of the medullary tube in the middle line, lying in the dorsal septum. 

 Immediately after its origin its calibre abruptly diminishes almost to vanishing point 

 and in this attenuated portion there are no nuclei. This root likewise has a back- 

 ward course. When nearly half-way between the medullary tube and the epidermis, 

 its diameter resumes the normal size and a central lumen appears. 



Series ii. The first root has a hollow origin and is very slightly oblique, running 

 almost directly from medullary tube to epidermis, so that in one section it is united 

 to the former at its proximal and to the latter at its distal end. It is not a simple 

 cylindrical root but is triangular in shape, the apex of the triangle being its inser- 

 tion in the epidermis, and the base of the triangle its elongated origin from the 

 medullary tube. The anterior side of the triangle is directed backwards and the 



' Series i — v refer to series o£ sections through different specimens which do not necessarily correspond with 

 those given on p. 231. 



