264 ENTEROPNEUSTA FROM THE SOUTH PACIFIC, 



projects forwards over the last pair of gill-pockets as a very short, free, blindly-ending 

 tube." 



Here the resemblance ends, for in Pt. ruficollis the walls of the diverticulum 

 instead of being slightly folded as they are in Pt. hedlei/i [Hill, I. c. PI. XXII. 

 Fig. 11], are thrown into the most complicated folds, so that the cavity is greatly 

 subdivided and in section appears as a multiple lumen (PI. XXX. Fig. 33). Its com- 

 munication with the gut extends over a comparatively short distance, in fact there is 

 little more than an elongated orifice of communication behind which it is again pro- 

 duced backwards, for a relatively long distance, as a coecal tube tapering slightly towards 

 its posterior extremity. The lumen ceases some distance in front of its posterior end, 

 and the structure is then a solid mass of densely nucleated tissue. 



In mature specimens, the gonads actually penetrate into that portion of the peri- 

 visceral cavity which occurs between the attenuated free posterior end of the post- 

 branchial canal and the dorsal wall of the gut. 



The postbranchial canal of Pt. ruficollis, as here described, appears to me to 

 present the characters of a vestigial structure for which I will at once proceed to 

 offer an explanation, a certain amount of repetition being unavoidable. 



Pt. flava. 



1. The pharynx varies greatly in length. 



2. The postbranchial canal is the direct continuation of the branchial portion of 

 the gut ; it is neither produced anteriorly nor posteriorly as a coecal tube. 



3. The terminal gill-clefts occur, and new ones arise in normal succession at the 

 dorsal sides of the postbranchial canal. 



4. The walls of the postbranchial canal are smooth, and its cavity is throughout 

 in free communication with the ventral division of the gut. 



Pt. ruficollis. 



1. The length of the branchial region is approximately constant, as shown by a 

 dozen specimens ; and it is characteristically short. 



2. The postbranchial canal is not in direct continuity with the branchial portion 

 of the gut; it is produced anteriorly and posteriorly, as a coecal tube, beyond the 

 region of its communication with the cavity of the gut. 



3. The terminal gill-clefts do not communicate with the postbranchial canal, but 

 are quite separated from it, occurring in the dorsal wall of the ventral division of 

 the gut below and beside the postbranchial canal. 



4. The walls of the postbranchial canal are thrown into complex folds, by which 

 its cavity is subdivided ; it only communicates over a short stretch with the gut, 

 and even then the orifice of communication may be interrupted ; towards the posterior 

 end of its free backwardly produced portion the lumen is obliterated, and replaced by 

 a solid mass of tissue with densely packed nuclei. 



