ON THE DIPmOHORDA. 723 



sac, glomerulus, and subneural glauJ protrude forwards 

 into the pre-oral cavity, and hence are covered dorsally as 

 well as ventrally; but in Ceplialodiscus they protrude 

 upwards between pre-oral cavity and collar cavities, 

 and they are therefore dorsal and posterior to the former. 

 In this Avay the pericardial sac lies in contact with the dorsal 

 ectoderm, and the subneural gland is only separated there- 

 from by the dorsal sinus. This difference cannot be regarded 

 as fundamental in view of the anatomical resemblance, and 

 we have seen above that it is due to the forward protrusion 

 into the pre-oral cavity of the subneural gland in Balano- 

 glossus, whereas in Cephalodiscus it remains in the 

 collar. 



Of other points the homology of the subneural gland is a 

 most important question. It appears desirable to adhere to 

 this term, firstly, because it is unquestionably glandular in 

 function; secondly, because it has precisely the same re- 

 lationship to a system of dorsal and ventral grooves in 

 the pharynx as is the case with the similarly-named organ 

 in Tunicata (10); and thirdly, because its anatomical position 

 is exactly under the main nerve-mass. These and other 

 facts led me to doubt its homology with the " Eicheldarm ^' 

 of Balano gloss us, but its relationships to pericardial sac 

 and glomerulus and the chordoid structure of its apex 

 appear to me to be conclusive in favour of accepting Har- 

 mer's original comparison. I would extend the appellation 

 of subneural gland to the organ in Balanoglossus, for, ns 

 in so luauy other features, Cephalodiscus would appear to 

 show us a more primitive condition of the organ than Balano- 

 glossus. In making this comparison it appears to me to be 

 questionable how far the subneural gland is at all comparable 

 to the notochord of the Vertebrata. As indicated elsewhere 

 (11, p. 412)j a chordoid histological structure by itself 

 cannot be regarded as an absolute criterion of homology, and 

 the occurrence of chordoid organs of the same nature as, but 

 not homologous with, the Vertebrate notochord is to be 

 expected in these low chordates. The view of Willey that 



