254 



ENTEROPNEUSTA FROM THE SOUTH PACIFIC, 



Branchiogenital Transition. 



The external features of the branchiogenital transition are shown in PI. XXVII. 

 Fig. 6. The branchial tract assumes an elevated or vaulted form and the dorsal 

 nerve-cord causes a crest-like projection. 



The pharynx is succeeded by a thick-walled postbranchial canal similar in all 

 essential respects to the corresponding structure in Pt. flava (PI. XXX. Fig. 23). This 

 structure has not previously been recorded in a member of the subgenus Tauroglossus. 

 Its walls are formed of high, closely-packed, ciliated columnar cells, with nuclei at different 

 levels, but especially crowded in the central portion of the epithelium. This postbranchial 

 canal has a narrow lumen open continuously below into the main cavity of the gut. 

 The last pair of gill-slits occurs at its dorsal borders as in Pt. flava. In front it 

 constitutes a high median crest or duplication of the gut-wall which, posteriorly, 

 becomes lowei- and lower until it disappears, and its place is then taken by an 

 ordinary duplication of the gut-wall lined by normal gastral epithelium consisting of 

 low, cubical, ciliated cells with basal nuclei. 



The lateral septum arises on each side of the postbranchial canal behind which 

 it arises from the dorso-lateral borders of the gut. In front of the postbranchial 

 canal, that is, in front of the last gill-slit, both the origin and the insertion of the 

 lateral septum occur in the basement-membrane of the epidermis. Its insertion marks 

 the position of the gonaducal line which lies on the inner side of the genital 

 pleura near the base, but separated by a wide interval from the branchial groove. 

 Contrary however to what takes places in Pt. flava, the lateral septum only extends 

 for a short distance into the branchial region and is by no means coextensive with 

 the genital pleurae. In front of the lateral septum the gonad ucal line is denoted 

 by a fold of basement-membrane carrying a lateral blood-vessel ; and moreover, this 

 lateral blood-vessel occasions or is associated with the same interruption of the longi- 

 tudinal musculature which accompanies the insertion of the lateral septum itself. 

 Although, as mentioned above, the gonads do not, in this species, extend to the 

 anterior end of the genital pleurae but commence two or three millimetres behind 

 the posterior rim of the collar, nevertheless the gonaducal line, as defined by the 

 presence of a lateral vessel and by the interruption of the longitudinal musculature, 

 is continued beyond the anterior limit of the gonads to the anterior extremity of 

 the genital pleurae. 



Thus, although neither the gonads nor the lateral septa are coextensive anteriorly 

 with the genital pleurae, the gonaducal line is. We have here, therefore, evidence of a 

 recession of the gonads from the anterior end of tlie trunk. 



In immature specimens the medial and lateral branches of the gonads, in the 

 posterior branchial and genital region, abut simply upon the lateral or gonaducal line 

 as shown in Fig. 23, PI. XXX. In a mature female such as the one represented in 

 Fig. 6, PI. XXVI. accessory genital ducts occur laterally from the main series. It seems 

 quite certain that the subdivision of the gonads which accompanies the appearance of 

 accessory ducts, in Pt. carnosa, is simply due to growth and constriction from the 

 original gonad ; not to the formation of independent accessory gonads. There are no 

 accessory ducts mediad of the gonaducal line, but the medial branches of the gonad 



