WITH NOTES ON THE WEST INDIAN SPECIES. 263 



TRUNK. 



Branchial Region. 



The character of this region is shown in PI. XXVII. Fig. 7, and in section in 

 PI. XXX. Fig. 31. 



The chief feature about it, namely, its shortness, has been already alluded to. Each 

 half of a gill-cleft is crossed by 10 — 12 synapticula. 



The lateral septa only extend for a short distance into the posterior portion of 

 this region, namely, to the anterior border of the posterior depression of the branchial 

 grooves, described below. 



Branchiogenital transition. 



At the posterior end of the branchial region, the branchial grooves undergo an 

 abrupt and deep depression at the base of which the posterior gill-clefts open (PI. 

 XXVII. Fig. 7, and PI. XXX. Fig. 32). A similar depression of the branchial grooves 

 has been described by Hill in Pt. hedleyi. In Pt. ruficollis the depression is localised 

 in the posterior end of the branchial region. In its deepest portion it bears a strong 

 resemblance to the dermal pores which I have described in Spengelia porosa (see below, 

 p. 275) into the base of the most anterior of which the posterior gill-slits likewise open. 



At the posterior end of the branchial region the dorsal wall of the pharynx, 

 i.e. the epibranchial ridge or band, sinks deeper below the surface and the height of 

 the dorsal mesentery is correspondingly increased (PI. XXX. Fig. 32). By this sinking 

 inwards of the epibranchial ridge, the branchial division of the pharynx is reduced 

 to zero, and the last pair of gill-pouches appear as small diverticula on each side 

 of the epibranchial ridge, continuous with which a longitudinal ciliated groove passes 

 back for some distance into the anterior portion of the genital region at the base of 

 the postbranchial canal (PI. XXX. Fig. 33). 



The postbranchial canal of Pt. ruficollis differs greatly from the corresponding- 

 structure of Pt. fiava and Pt. carnosa, both in its relations to the gill-slits and in 

 its general character. In the two last-named species we have seen that the post- 

 branchial canal is in direct continuity with the branchial portion of the gut, and that 

 the terminal gill-clefts occur at its summit. 



In Pt. ruficollis, on the contrary, the branchial division of the gut comes to an 

 abrupt end, and the postbranchial canal appears as an independent diverticulum of 

 the gut, while the terminal gill-clefts together with the longitudinal grooves continued 

 behind them in the dorsal wall of the gut, lie at the base, instead of at the summit 

 of the postbranchial canal. The relations here described are not only important in 

 providing a clue as to the meaning of the postbranchial canal, but they are perhaps 

 of even greater interest in furnishing a striking example of readjustment of topo- 

 graphical relations of gill-clefts. 



The postbranchial canal of Pt. ruficollis projects forwards for some distance beyond 

 the region of its communication with the gut, as a coecal tube, resembling, in this 

 respect, the condition described by Hill in Pt. hedleyi. With regard to the latter 

 species Hill says (loc. cit. p. 342): — "At its anterior end the dorsal diverticulum 



w. in. 37 



