262 



ENTEROPNEUSTA FROM THE SOUTH PACIFIC, 



(VII) unites the nerve-cord directly with the epidermis. VIII is band-like. IX is 

 slender and arises a moderate distance behind VIII. 



As the medullary tube nears the posterior portion of the collar-region it approaches 

 the epidermis more and more, and the dorsal mesentery becomes progressively shorter. 

 The consequence is that root X is very short and amounts to little more than a 

 direct fusion of the nerve-tube with the epidermis. XI is quite slender and runs 

 obliquely backwards, being followed by a twelfth root (XII) of similar character. 



Series ii. The first root is band-like in its basal half and subcylindrical distally. 

 It commences in front of the bifurcation of the nuchal skeleton, while its radical 

 portion extends backwards in continuity with the nerve-cord beyond the bifurcation. 

 It has a winding course in the dorsal septum shortly behind the free anterior margin 

 of the latter. 



The second root (II) has a band-like or crest-like origin ; III is inclined forwards ; 

 IV has the vestige of an axial lumen at its base ; V also has the vestige of an 

 axial canal in continuity with the central canal of the medullary tube ; VI has the 

 merest trace of a basal diverticulum from the central canal ; VII, VIII, and IX arise 

 in close succession and are quite solid ; a long interval occurs before X closely followed 

 by XI is given off ; XII has a horizontal course backwards, and is apparently without 

 fibres. 



About this region {i.e. in the hinder third of the cord) the central canal of the 

 medullary tube has very irregular walls in the specimen under consideration. After 

 another long interval a small root XIII occurs. XIV has interrupted vestiges of an 

 axial lumen, but I could not trace this root continuously to the epidermis, and in fact 

 I think it anastomoses with XV, which in its turn does not reach the epidermis but 

 passes back to XVI which does. A much reduced root XVII occurs but does not reach 

 the epidermis. There is a somewhat doubtful vestige of an eighteenth root which is 

 immediately followed by the fusion of the nerve-cord and epidermis at the lip of the 

 posterior neuropore. 



Collar-canals and pores. 



This species is particularly interesting in respect of its collar-canals in that it 

 appears to afford a clue as to the origin of the dorsal plication which is such a 

 frequent feature of the canals. In section this dorsal plication looks temptingly like 

 a tongue-bar and the possibility of the collar-pores being modified gill-slits has been 

 referred to by Morgan. It is therefore a matter of some importance to show con- 

 clusively that the dorsal plication is in no sense comparable to the tongue-bar of a 

 gill-cleft. It is in fact simply due to the fusion of the infolded edges of the collar- 

 funnel ; a distinct raphe is discernible throughout almost the entire extent of the 

 lappet. 



The external aperture of the collar-canal, i.e. the collar-pore, opens as usual into 

 the first gill-pouch. The dorsal plication projects beyond the limits of the collar-pore 

 as a valve-like structure overhanging the branchial groove as far back as the second 

 gill-pore (PI. XXX. Fig. 30). 



