236 ENTEROPNEUSTA FROM THE SOUTH PACIFIC, 



Pt. flava is to be interpreted as an incomplete ventral septum ; but there is reason for 

 supposing that it would be more correct to treat the ventral vascular complex and the 

 ventral septum as two distinct structures which may or may not, coincide. 



Collar canals and Pores. 



The collar-canals (Kragenpforten) of Pt. flava vary so much in their length and 

 in the character of the dorsal wall, that they are of little use for diagnostic purposes. 

 The dorsal wall is folded into the lumen of the canal by a simple tongue-like pli- 

 cation. This dorsal fold is always deep, but is larger in some cases than in others 

 and resembles roughly the condition described and figured by Spengel for Pt. minuta 

 and Pt. aperta. 



Each canal opens internally into the collar-coelom by a ciliated semilunar funnel. 

 Sometimes the canal is so short that its ventral wall is fused with the epithelium of 

 the first gill-pouch in the same transverse plane with the funnel. More usually a tube 

 of some length with deeply infolded dorsal wall intervenes between the internal funnel 

 and the external pore. The latter opens into a special dorsal section of the first gill- 

 pouch. The first gill-slit of all Enteropneusta (unlike Amphioxus) is complete, and 

 provided with a tongue-bar like the rest. The first septal bar is therefore confluent 

 with the epithelium of the throat or collar-gut. The collar-pore opens into the dorsal 

 angle made by the first septal bar with the posterior edge of the collar, this angle 

 being tucked in for some distance beneath the collar-rim (PI. XXVIII. Fig. 5). 



There are therefore three portions of a collar-canal to be considered, namely, (1) the 

 funnel, (2) the tube, and (3) the pore. The tube appears to be largely formed by 

 intercalary growth, between the funnel and the pore, during the life of the animal. 



TRUNK. 

 Branchial Region. 



I hope by this time that the fact of the existence of Enteropneusta with a free, 

 exposed pharynx, has sunk into the mind of the reader (PI. XXVI. Figs. 1 and 2). 



In horizontal sections it appears that the first two gill-clefts do open into gill- 

 pouches owing to the protrusion of the anterior end of the pharynx within the 

 posterior limits of the collar, but the bulk of the gill-slits open freely to the exterior, 

 a fact which might also be expressed by saying that the gill-pouches are confluent. 



In sections through this region Pt. flava is apparently distinguishable from the 

 other two species of the subgenus Chlamydothorax, as described and figured by Spengel, 

 in respect of the relative cubic capacity of the branchial and oesophageal portions 

 of the pharynx. In Pt. erythraea the oesophageal division predominates over the 

 branchial division ; in Pt. bahamensis the two divisions are nearly equal ; finally, in 

 Pt. flava the branchial predominates over the oesophageal division (PI. XXVIII. Fig. 6). 



The line of demarcation between the bases of the gill-bars and the oesophageal 

 epithelium is occupied, as in all Ptychoderidae, on each side by a prominent longi- 



