230 



ENTEROPNEUSTA FROM THE SOUTH PACIFIC, 



the limits of the proboscis-cavity as to entirely displace the lateral portions of the 

 cavity, thus giving rise to two pairs of canals, namely, the dorsal canals and the 

 ventral canals (cf. PI. XXX. Fig. 25). The former are associated with the proboscis- 

 pores and the latter terminate in the ventral coecum of the proboscis (PI. XXVIII. 

 Figs. 2—3). 



As indicated above, the partition between the ventral canals is formed by the ventral 

 septum of the proboscis, which, as in most but not in all species of Enteropneusta, 

 has free anterior and posterior borders*. The septum dividing the dorsal canals is 

 formed by the dorsal wall of the pericardium (Herzblase) which reaches the basement 

 membrane of the epidermis (cf PI. XXX. Fig. 25 h). 



The anterior border of the ventral septum is nearly vertical, usually with a slight 

 backward inclination, but no doubt this inclination would vary under different conditions 

 of contraction. The septum extends a short distance in front of the region of the 

 coecal dilatation of the stomochord, but stops far short of the anterior end of the latter. 



In front of the septum, the stomochord is held in position largely by the median 

 dorso-ventral muscles of the proboscis. 



Proboscis-pores. 



Pt. fiava is distinguished by the constant occurrence of paired proboscis-pores ^ 

 which, however, differ from one another in their relations to the coelom of the pro- 

 boscis. As I attach great importance to these structures I will describe my observations 

 in some detail. 



The proboscis-pores, by which the dorsal proboscis-canals discharge to the exterior, 

 are interesting in this species on account of the variations which they exhibit^. It is 

 important, however, to bear in mind the fact that a dorsal canal does not lead directly 

 to the corresponding proboscis-pore, but communicates first with a terminal bladder-like 

 dilatation lined by ciliated columnar epithelium. The communication between the canal 

 and its terminal vesicle is effected by the intermediation of a narrow connecting tube, 

 which proceeds from the posterior dorsal angle of the coelomic canal. There are, therefore, 

 four structures to be considered, namely, (1) the dorsal coelomic canal, (2) the connecting 

 tube (coelomic pore), (3) the terminal (ectodermal) vesicle, and (4) the proboscis-pore. 

 The terminal vesicle is the Eichelpforte of Spengel, who identifies it with the ciliated 

 excurrent canal or pore-canal of the anterior coelomic vesicle (Wassersack) of Tornaria. 



I now pass on to a selected serial account of my observations on these structures, 

 based on serial sections through different individuals. 



1 Spengel has shown that there is no posterior free border of the ventral septum in Glandiceps and I find 

 the same condition in Spengelia. 



^ Mr J. P. Hill, who has himself made some observations on Pt. fiava to which I shall have occasion to 

 refer, first saw the paired proboscis-pores of Pt. fiava in preparations of his own made from material collected 

 by Mr Charles Hedley in Funafuti. 



3 In some other species, e.g. Pt. minuta [Spengel] and Pt. australierisis Hill, the proboscis-pores vary 

 greatly but not in the same way as those of Pt. fiava. In Pt. hedleyi, Hill has described paired proboscis- 

 pores which open nearly or quite coincidently in the middle line. 



