WITH NOTES ON THE WEST INDIAN SPECIES. 



253 



than a cylinder of basement-membrane ; the cellular contents expand in the tract of 

 the root which intervenes between the points of origin and insertion. 



Finally, in the specimen now under consideration, the dorsal septum of the collar 

 is lacking. 



Collar-canals and Pores. 



The collar-canals have a characteristically folded epithelium (PL XXX. Fig. 20). 

 They open, on each side, into the first gill-pouch dorso-laterally with respect to the 

 first gill-pore. It is 'a striking fact that the first gill-pore itself opens, together with 

 the collar-pore, into the posterior end of the medullary tube immediately in front 

 of the posterior neuropore, and in front of the posterior commissural ring-nerve of the 

 collar (PI. XXX. Fig. 21). 



In another specimen these relations were not so striking, the opening of the first 

 gill-slit occurring slightly farther back at the level of the posterior neuropore'. 



It is probably not a matter of very great importance whether the first gill-pore 

 occurs a few thousandths of a millimetre in front of or behind the level of the 

 posterior neuropore ; and the same remark applies to the proboscis-pore in its relation 

 to the anterior neuropore. What is perhaps of some importance is to take note of 

 the fact that while the proboscis-pore may be intimately associated with the anterior 

 neuropore, the collar-pores and first gill-pores may be similarly associated with the 

 posterior neuropore. 



TRUNK. 

 Branchial Region. 



The genital pleurae become reduced in height as they approach the posterior rim 

 of the collar, and they converge towards the middle line. The gonads do not extend 

 to the anterior end of the genital pleurae but commence some distance behind the 

 collar, as, indeed, is frequently the case (Spengel). 



Spengel has shown that in those species in which the gill-clefts open into gill- 

 pouches (i.e. in the majority of Enteropneusta) the gill-pouch is exactly as deep 

 (dorso-ventrally) and as broad (longitudinally) as the corresponding gill-slit, except in 

 Pt. gigas and Pt. clavigera, where the gill-pouches are produced ventrally below the 

 base of the gill-slits into deep coecal diverticula. In Pt. carnosa such ventral coeca 

 of the gill-pouches are also present and of great depth anteriorly, becoming shallower 

 posteriorly (PI. XXX. Fig. 22). At the posterior end of the branchial region the gill- 

 pouches are very capacious in the transverse direction. 



The tongue-bars are united to the corresponding septal bars by more than 30 

 synapticula on each side. 



^ It might be said with equal justice and perhaps even with more accuracy that, in the specimen referred 

 to, the posterior neuropore occurred at a slightly more anterior level. 



