1 1 1 



While the embryos described above shew obvious indications of possessing the essential 

 structure of Cephalodiscus, the clue to the precise relations between embryo and adult will 

 probably be furnished by the study of the later stages, which are not yet forthcoming. The 

 consideration of the embryo of C. levinseni shewn in fig. 200 suggests that the ventral thickening 

 can be used as a crawling organ, and that it is probably already in a condition to secrete the 

 commencement of the coenoecium. The absence of a functional alimentary canal further points 

 to a free life of brief duration, and it appears probable that the larva constructs its tube after 

 a very short free life, and undergoes an important part of its metamorphosis within its tube. 

 The very striking folding of the ectoderm in the old embryos of C. levinsein is perhaps a 

 preparation for the stretching of the body-wall necessary to provide accommodation for the 

 future alimentary canal, the anal region of which appears to be indicated in fig. 210. 



One of the points which appears to be most problematical in the structure of the 

 embryos is the meaning of the anterior umbrella-like ma.ss of yolk. It is conceivable that in 

 the rearrangement of the viscera which presumably accompanies the metamorphosis, it might give 

 rise to part of the anterior end of the alimentary canal. That conclusion seems to be improbable 

 if the part in question is really in front of the collar-cavities (cf. figs. 209, 210). 



Another possibility which must not be left out of account is that the embryo undergoes 

 no metamorphosis, but that it may fix and give rise to the adult zooids by budding. A process 

 of this kind is characteristic of the great majority of Ectoproct Polyzoa, in which (with rare 

 exceptions) the alimentary canal of the free larva is represented by at most a mass of yolk- 

 like material, as in Cephalodiscus. Although this possibility can hardly be excluded, it seems to 

 me that the embryos of Cephalodiscus possess sufficient indication of the adult organs - — and 

 particularly of the body-cavities — to make it probable that the embryo undergoes a meta- 

 morphosis into a zooid. 



It is not impossible that the umbrella-like mass of yolk represents in itself the splanchnic 

 epithelium of the anterior body-cavity. While the paired cavities, in some of the specimens at 

 least, are unmistakeably complete coelomic sacs, I have not been able to convince myself in 

 any case that the anterior body-cavity has a splanchnic epithelium distinct from the yolk-mass 

 itself. I am in fact inclined to accept this explanation as the most likely one which can at 

 present be given. This interpretation is in accordance with the account given by Bateson 

 (84, p. 218) of the development of Dolichoglossus kozvalevskii^ a species of Balanoglossus in 

 which the development is direct. It appears from Bateson's account that the entire front end 

 of the archenteron is separated off as the anterior enterocoel, the walls of which overlap the 

 axial enteron in precisely the same manner, though to a less e.xtent, as the anterior yolk 

 overlaps the posterior yolk in Cephalodiscus levinseni (cf. Bateson's figs. 35, 40, 27, 28). 

 There are indeed differences in detail between the two cases. Thus in Balanoglossus, the 

 somatic layer is as thick as, or thicker than, the splanchnic layer, while in Cephalodiscus the 

 somatic wall is excessively thin, and at first barely recognisable, while the splanchnic layer is, 

 ex hypothesi, constituted by the thick yolk-mass itself. Again, while in Bateson's larva, the 

 anterior enterocoel communicates at first with the enteron, in CepJialodiscus the whole complex 

 forms a solid mass of yolk without any cavities. 



