WITH NOTES ON THE WEST INDIAN SPECIES. 271 



the intimate relation between the substance of the nuchal skeleton and the chondroid 

 tissue, are the salient features in the nuchal skeleton of this species (PI. XXXI. Fig. 

 39). The cornua of the skeleton, as already mentioned in the classification, extend to 

 the posterior region of the collar (PI. XXVIII. Fig. 1 d). The keel diminishes poste- 

 riorly, ceasing entirely some distance in front of the point of bifurcation ; the body of 

 the skeleton has then (i.e. behind the keel) a cubical shape with rounded edges. 



Proboscis-pore. 



The right dorsal canal ends blindly, while the left communicates with an end- 

 vesicle which opens to the exterior by a narrow pore on the left side (PI. XXXI. 

 Fig. 39). The pore is very long, slit-like and approximately co-terminous with the 

 end- vesicle; in both of which features the present species differs from S. alba. The 

 right canal has a narrow dorsal canalicular extension corresponding to the place where 

 it would communicate with an end-vesicle, were one present on that side. In the 

 vicinity of the point of communication between the left canal and its end-vesicle, there 

 are muscular fibres about the base of the vesicle which appear as if they would act 

 as a sphincter. 



COLLAR. 

 Collar Nerve-cord. 



The medulla of Spengelia porosa does not contain a continuous central canal but 

 a large number of small disconnected medullary cavities arranged quite irregularly (PI. 

 XXXI. Fig. 41). 



The anterior neuropore leads into a broad, transverse, median lumen which soon 

 gives place to a series of lateral cavities at each side of the nerve-cord. In addition 

 to these lateral cavities there are other smaller cavities scattered about in the sub- 

 stance of the cord. Here and there cavities seem to be entirely absent and the cord 

 at such places appears quite solid in section. 



In the neighbourhood of the medullary cavities there occur numbers of deeply 

 staining mucous cells, not unlike those found in a similar position in relation to the 

 fragmented lumen of the stomochord. 



The nerve-fibre layer is confined to the ventral side and to the lateral margins 

 of the cord, but is absent from the median dorsal tract. 



The collar cord of the present species is remarkable for the presence of at least 

 one vestigial root. It is no new thing for a root not to reach the epidermis but 

 it is new for it to behave as it does in S. porosa (PI. XXXI. Figs. 40 a — 40 c). 



The vestigial root which claims special attention occurs in the region of the 

 bifurcation of the nuchal skeleton. It arises from the non-fibrous dorsal side of the 

 cord slightly to the right of the middle line. It bends first to the right and then 

 runs forward for an appreciable distance, without however reaching the basement-mem- 

 brane of the epidermis. On the contrary, it abuts upon and terminates in a re- 

 latively dilated vesicle, the walls of which are crowded with some mucoid substance 



w. in. 38 



