with notes on the west indian species. 291 



Proboscis-pore. 



Only the left dorsal canal communicates with an end-vesicle ; the latter, however, 

 does not end simply at the pore but becomes subdivided by a duplication of the 

 wall into two unequal portions, a smaller right and a larger left moiety. Thus the 

 effect is produced of two pores opening by a common median orifice (PI. XXXII. 

 Fig. 6'4). It seems quite obvious that the smaller dextral portion of the end-vesicle 

 corresponds with the dextral vesicle of Pt. flava. The slit-like pore of the left portion 

 is longer than that of the right, but both portions of the end-vesicle have long post- 

 trematic coecal extensions (the left rather longer than the right) which lie on either 

 side of the forward extremities of the perihaemal cavities. The smaller dextral portion 

 maintains a more dorsal position than the bulk of the larger sinistral portion. The 

 latter extends backwards beyond the region of the anterior neuropore. 



Thus there is, in effect, a dextral end-vesicle and a dextral pore without any 

 vestige of connection with the right dorsal proboscis canal. The pronounced sub- 

 division of the end-vesicle in this species throws light upon the less complete sub- 

 division observed in Pt. carnosa. The peculiar conditions here described are still more 

 clearly established in the species next dealt with. 



COLLAR. 



Nerve-cord. 



There is no median anterior neuropore in this species; the medullary cord closes 

 in solid in the anterior median tract but there are two short lateral cavities bounded 

 by numerous mucous cells (PI. XXXII. Fig. 65). These paired cavities open in front 

 and are essentially due to the backward continuation of the dorso-lateral angles made 

 by the union of the neck of the proboscis with the collar flap. This is a clear and 

 instructive example of the way in which a median structure can assume a paired form. 



The main body of the collar nerve-cord is practically solid, the medullary cavities 

 being reduced to the merest vestiges. It is also characterised, in its anterior third, 

 bv the presence of a large quantity of yellowish flocculent tissue, the bulk of which 

 forms a tract on each side of the middle line. In front, the cord is sub-triangular 

 in section, the ventral angles being produced downwards so as to form bold projections 

 into the perihaemal cavities; behind the orifice of the stomochord, these ventral horns 

 of the medulla flatten out and the cord then becomes a transversely elliptical band. 



The first and only root is massive and sub-solid ; it has a backward course along 

 the anterior free edge of the dorsal septum and occurs in front of the orifice of the 

 stomochord. 



The dorsal septum only extends for a short distance behind the root, when it 

 becomes interrupted and finally disappears only to reappear near the posterior end of 

 the collar in the region of the well-defined wide posterior central cavity of the nerve- 

 cord which duly opens by the posterior neuropore. 



