ON THE DIPLOOHOEDA. 355 



thin untnetaTTiorphosed layer of protoplasm in the notochord 

 of Actinotrocha, can be traced to the fact that only un- 

 developed stages are found. The notochord of the young 

 Phoronis most nearly approaches that of Cephalodiscus in 

 structure. 



The same supposition as was put forward in the case of 

 Actinotrocha will hold for the comparison of Cephalo- 

 discus and Balanoglossus. It seems likely that in Bala- 

 noglossus the two notochords have protruded still further 

 forward into the proboscis and have fused in the middle line, 

 and that the two thickened masses of mesoblastic skeletal 

 tissue (chondroid) in Cephalodiscus have met in the mid- 

 ventral line in Balanoglossus to form the skeletal rod. 



PI. 26) fig. 20, gives a diagrammatic sagittal section of the 

 front end of Cephalodiscus, comparable to those of Balano- 

 glossus and Actinotrocha, previously given. A compari- 

 son of the three figures will show the close similarity between 

 the three types. 



The only previous observation, so far as I am aware, of the 

 chordoid tissue of Cephalodiscus is in the work of 

 Mcintosh (2), in which, referring to the walls of the pharyngeal 

 clefts, which are also of chordoid character, he speaks of " the 

 translucent wall of the slits which seems to be a modified con- 

 tinuation of the pharyngeal mucous membrane. The granules 

 are finer, and the whole tissue is more translucent " (p. 16). 



As a matter of fact, the walls of the pharyngeal cleft present 

 identically the same histological structure as those of the noto- 

 chords, except that the vacuoles are smaller. The chordoid 

 tissue of the notochord is continued directly into that of the 

 pharyngeal cleft (fig. 16), and meets the ectoderm at the distal 

 end of the cleft. In fig. 19 the left-hand end of the notochord 

 is continued into the commencing pharyngeal cleft. 



Fig. 18 is a transverse section of the cleft, drawn from a 

 section of the same series as, and a little further out than, 

 fig. 14. Just as the notochords in the pharynx subserve a 

 supporting function, so here the chordoid tissue of the gill-slits 

 evidently has also a similar function analogous to that of the 



