REPORT ON CEPHALODISCUS DODECALOPHUS. 45 



'2. Existence of au uupaired body-cavity in the proboscis, and of paired cavities in 

 the collar and in the trunk. 



3. Proboscis-pores (pau'ed in Balanoglossus kupfferi), opening into the body-cavity 



of the prae-oral lobe. 



4. Collar-pores in similar relation to the collar-cavity, their external apertures being 



overhung by an operculum developed from the coUar. 



5. Gill-slits (one pair for a considerable period in the embryonic historj- of Balano- 



glossus) ; their relation (in Balanoglossics, the relation of the first pair) to the 

 operculum and to the external apertures of the collar-pores. 



6. Existence of a notochord as a diverticulum of the alimentary canal, growing 



forwards into the proboscis-stalk. 



7. Dorsal central nervous system, most richly developed in the collar, but 



extending on to the proboscis ; the fact that the nervous tissue lies in the 

 epidermis. 



Before leaving this subject, it will be well to refer briefly to the highly interesting 

 pelagic larva of Balanoglossus, discovered by Weldon in the Bahamas.* A noteworthy 

 feature of this larva is the development of a series of tentacles arranged in six grooves 

 passing, equidistant from one another, in a longitudinal direction along the surface of 

 the prse-oral lobe. Although the tentacles are not in the same position as those of 

 Cephalodiscus, it is a suggestive fact that this larva affords another case of the 

 development of tentacles in the anterior part of the body in Balanoglossus or its allies, 

 and it is at least possible that their appearance in the Tornaria may be due to a process 

 of reversion or atavism. 



It a^jpears to me that whatever may be thought of any single similarity between the 

 two genera given in the above list, the cumulative evidence of the whole sequence t f 

 resemblances points irresistibly to the conclusion that Cephalodiscus and Balanogloss^is 

 are near allies, and I would propose to remove CephalodisciLS from its previous position 

 amongst the Polyzoa, and to place it definitely as a second genus in Bateson's group of 

 the Hcmichordata. The character of the Vertebrate features of Cephalodiscus (notochord, 

 gill-slits, nervous system) appears to justify an approximation of this genus to Balano- 

 glossus in particular rather than to any other group of the Chordata. 



The most important difference between Cephalodiscus and Balanoglossus appears to 

 mo to consist in the relations of the dorsal and ventral surfaces in the two genera. The 

 difference is, however, a non-essential one. Whilst in Balanoglossus the elongation of 

 the embryo takes place in the line of its long axis, the ventral elongation of a similar 

 embryo in a line at right angles to its primitive long axis would give rise to the condi- 

 tion found in Cephalodiscus. We may suppose that the stalk has originated in this 



' Vidt Proe. Eoy. Soc, voL iJii. p. 146, 1887. 



