262 ENTEROPNEUSTA FROM THE SOUTH PACIFIC, 



(VII) unites the nerve-cord directly with the epidermis. VIII is baud-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 w T ith 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). 



