ON THE DIPLOCHORDA. 351 



of contractile walls, are j^iven off to the gonads, and there are 

 traces of a ventral vessel which also passes out into the pedicle 

 (fig. 17, V. b. v.). There is a hsemal vessel in each pinna of the 

 tentacles, but I have not traced these further (see " Branchial 

 plumes ^'). 



Many of these vessels have undoubtedly shrunk in size, and 

 in some cases lost all lumen ; so that in the living animal, 

 especially if the blood be tinted, they probably form a very 

 prominent system. The same condition is met with in pre- 

 served Actinotrocha, in which the vessels are difficult to 

 trace, though followed with ease in the living larva. 



The vascular system here described corresponds, even to 

 minute detail, with that of Actinotrocha and Phoronis. 



Subneural Gland. ^ — In his comparison of this species 

 with Balanoglossus, Harmer(l) drew up a list of features in 

 which it closely resembled the latter. Amongst these he com- 

 pared an anterior diverticulum of the gut to the notochord of 

 Balanoglossus. Whilst confirming the other features he 

 described, and further adding thereto the very similar vas- 

 cular system, I think that the homology of this organ to the 

 enteropneustan notochord (or to that of any other animal) 

 cannot be maintained. Its origin (from epiblast or hypoblast) 

 cannot be determined till the sexual development is followed, 

 for I have every reason to believe that the entire gut in the 

 young buds is formed from the ectoderm of the parent. It 

 presents no histological features resembling those of every other 

 notochord yet described, and it is difficult to conceive of its 

 performing any supporting function. As already suggested, 

 I propose to homologise it with the subneural gland of 

 Actinotrocha (and in part to the proboscis vesicle of Bala- 

 noglossus.^ 



In view, however, of its " notochord ^^ claims, I have worked 



' Described here to follow ' vascular system,' though of course not meso- 

 dermal. 



* In section the subneural gland consists of a single layer of elongated 

 ciliated cells surrounding a small lumen, and in many cases there may be seen, 

 in the centre of the lumen, a rod of hardened darkly staining mucus. 



VOL. 40, PART 2. — NEW SEK. B B 



