WITH NOTES ON THE WEST INDIAN SPECIES. 



261 



over which its lateral portions arch. The dorsal median portion projects into the base 

 of the reduced stomochord, and may be described as cristate'. 



Behind the level of the posterior edge of the ventral septum the body of the 

 skeleton begins to alter its form. Its basal angles cease and their place is taken by 

 the adventitious skeletal elements which constitute the alary processes (PI. XXX. 

 Fig. 27). Farther back these come together and unite to form the prominent keel 

 which coexists in this species with the projecting lobe formed by the ventral coecum 

 of the proboscis (PI. XXX. Fig. 28). 



COLLAR. 



In respect of musculature and vascular system of the collar, the present species 

 conforms to the Ptychoderoid type. 



Collar Nerve-cord and Roots. 



There is a continuous medullary canal in the collar nerve-cord of Pt. ruficollis 

 which agrees therefore in this respect with Pt. hedleyi Hill. The dorsal wall of the 

 neural canal is, as a rule, sharply delimited towards the lumen, while the inner surface 

 of the ventral wall is sometimes quite irregular and without a clear line of demarca- 

 tion. The central canal contains debris (PL XXX. Fig. 29). 



Pt. ruficollis is remarkable for the large number of roots which may be present. 



Series i. The first root (I) arises as a solid outgrowth from the dorsal wall very 

 close behind the anterior neuropore ; it is a sleuder root and has a winding course 

 backwards at the anterior edge of the dorsal septum ; it is doubtful whether this 

 root contains any nerve-fibres, although it reaches up to the epidermis. 



The second root (II) follows close upon the first and has also a long and winding 

 course, but is much stouter than the first. 



Root III arises at the level of the fusion of II with the epidermis. It runs 

 horizontally backwards for a short distance, separated from the medullary cord by its 

 own basement-membrane and by that of the cord itself, as well as by a thin layer 

 of mesenchymatous tissue. As it proceeds backwards its calibre increases until finally 

 it becomes connected for a second time with the wall of the medullary tube, and 

 from this point it has a direct course to the epidermis. This remarkable condition 

 would perhaps be more correctly expressed by saying that III does not approach the 

 epidermis but runs backwards and fuses with IV. This at least is my interpretation 

 of the matter. The fourth root, thus defined, is a broad sagittal band, not cylindrical. 



After an interval, root V is given off'; it has a direct, vertical course to the 

 epidermis and is a normal cylindrical root, solid like the rest. Root VI resembles V 

 in all respects. It is followed by another still longer interval, and then a stout root 



^ Few of these statements are absolute. In one ease the body of the nuchal skeleton between the cupule 

 and the alary processes presents in outline an exact replica of the preceding triradiate stomochord as seen 

 in section (PI. XXX. Pig. 26). In another specimen the dorsal crest-like portion was barely represented, the 

 dorsal side of the skeleton being concave with a very slight median crest. 



