446 



Thus the subneural gland of Cephalodiscus and the 'Eicheldarm' of 

 Balanofflossus occupy entirely different relationships to the surround- 

 ing organs in each case. If either or both of these organs had lost their 

 connexion svith their parent-tissue, the gut- wall, a phyletic migration 

 might be assumed (cf. notochord and dorsal bloodvessel of Chor data), 

 but this is not the case, and it appears to me that the homology of two 

 organs in such different relative positions is hard to prove. 



To proceed to Mr. H armor's second contention: 



2) 'The comparison of the median 'notochord' of Cephalodiscus 

 with the proboscis -vesicle or heart vesicle of Balaiioglossus is un- 

 tenable.' 



My expressed idea was that just as in the hypophysis of the Ver- 

 tebrata (its suggested homologue) the distal part is segmented off 

 as the pituitary body, so in Balanoglossus the distal part of the sub- 

 neural gland is represented by the proboscis- vesicle. 



Fis. 1. 



Fig- 2. 



Anterior Boiiy-ojtUy. 



EicUIJ 



Fig. 1. Transverse section of epistome of Cephalodiscus, through distal end of 

 subneural gland. 



Fig. 2. Transverse section through proboscis of young Balanoglossus, (After 

 Morgan.) 



The main arguments for this homology are these : 



The proboscis-vesicle has exactly similar relations to the anterior- 

 and collar-cavities and to their intervening blood-sinus as has the 

 distal part of the sub-neural gland in Cephalodiscus (cf. figs. 1 and 2). 

 It lies dorsally and anteriorly to the anterior body-cavity and, together 

 with the 'heart', fills up the space between this and the collar-cavities. 

 In these figures may also be noticed the great similarity of the sub- 

 neural sinus (fig. 1) to the heart (fig. 2) as figured by Morgan. 



In each case, a portion of the anterior body-cavity is surrounded 

 by the blood-sinus and nearly cut off from the rest. 



Should the homology be true, one might expect either in ontogeny 

 or in the anatomy of some of the group (Ent ero pneus ta) to find a 

 trace of the proximal end of the sub-neural gland. 



