260 ENTEROPXEUSTA FROM THE SOUTH PACIFIC, 



In another specimen the left dorsal canal opens into the end-vesicle at the posterior 

 end of the pouched or coecal region of the stomochord ; the vesicle accompanies the 

 body of the nuchal skeleton behind the cupule and opens by the sinistral pore at 

 the level of the alary processes and posterior to the ventral septum which does not 

 extend into the free lobe (PL XXX. Fig. 27). 



In spite of these differences, however, the posterior border of the proboscis-pore 

 is, in both cases, equally close to the insertion of the neck of the proboscis into the 

 dorsal wall of the collar, and hence, equally near to the anterior neuropore. 



Beyond the point of communication with the end-vesicle, the rest of the left 

 canal breaks up into the islets of the chondroid tissue; and the right canal does 

 the same. The chondroid tissue which was first described by Marion in 1S85 

 and is called by Spengel the secondary skeleton, is one of the most remarkable 

 elements in the organisation of these animals. In the present species it is poorly 

 developed, as is usual for Ptychoderidae. It attains its maximum development in the 

 Spengelidae. It needs little perspicacity to predict that when the theory of chondri- 

 fication is better understood, the importance of this chondroid tissue in the Entero- 

 pneusta will be more fully appreciated. 



The end-vesicle opens widely, like an exposed pit, as do the end-vesicles of 

 Pt. flava. In Pt. hedleyi, Hill has shown that both sinistral and dextral pores are 

 present, opening close together or by a common median aperture. 



Ventral Coecum of Proboscis. 



The affinities which bind together the different species of Enteropneusta intertwine 

 and overlap in the most perplexing manner. Thus, Pt. ruficollis differs from the other 

 species of its subgenus and agrees with those of the subgenus Chlamydothorax in the 

 mode of termination of the ventral coecum of the proboscis. 



This coecal prolongation of the proboscis-coelom is continued far behind the 

 posterior free edge of the ventral septum, and forms a large pro-eminent lobe which 

 projects into the buccal cavity like the racemose organ of Pt. flava (PI. XXVIII. 

 Fig. 1 c, and PL XXX. Fig. 27). In Pt. hedleyi, the ventral coecum of the proboscis 

 is stated by Hill to end blindly " in what appears to be simply the thickened basement- 

 membrane of the epidermis " below the body of the nuchal skeleton. 



In Pt. minuta it extends for a very short distance beyond the ventral septum 

 as a flattened sac (Spengel, Mori., Tat', ill. Fig. 30). 



Nuchal Skeleton. 



The anterior cupule of the nuchal skeleton in which the stomochordal coecum 

 rests, presents no reliable features of diagnostic importance. The body of the skeleton 

 following upon the cupule has, on the contrary, definite features characteristic of the 

 species. It has a triradiate form closely resembling in outline the mitre-shaped stomo- 

 chord which lies in front of it. It sits like a cap upon the ventral proboscis-canals 



