io8 



tissue traversed by excessively fine threads containing a few minute nuclei. The organ is connected 

 laterally with a part of what appears to be an epidermic nerve-plexus. This («. /.) is seen on 

 tlie right side of fig. 190, but it is seen also on the left side in other sections of the same 

 embryo. The intimate connexion of the transparent organ with the embryonic nervous system 

 su2feests that the or^jan it.self is a sense-organ : and both in structure and in its relation to 

 the nervous system it recalls the "pyriform organ" of the larvae of Ectoproct Polyzoa : — a 

 resemblance which need not indicate any affinity of Ccphalodiscus to the Polyzoa '). 



The five coelomic spaces of Cephalodisacs are probably indicated in fig. 190. The 

 proboscis-cavity [b. c}) has a somatic epithelium, in its anterior part at least, but 1 find no 

 evidence of the existence of a splanchnic layer. The collar-cavity {b. ^.'), on the leit side, has 

 a form and position which appear to be very characteristic of this species. It is somewhat 

 triangular in section, with its base applied to the ectoderm and its apex projecting into the 

 emargination of the yolk. On the right side (which passes through a more ventral region than 

 does the left side of the section), the collar-cavity is a small oval vesicle, lying more posteriorly 

 than the corresponding left cavity. A similar difference is noticeable in the third body-cavities, 

 the section cutting the lumen of the left cavity and the ventral wall of the right cavity. 



Fig. 196 represents a more dorsal region of ilie same embryo. It involves the dorsal 

 end of the posterior pit, which in this embryo has the form of a vertical slit, and it shews 

 the five body-cavities. The manner in which the paired cavities end ventrally has not been 

 ascertained with certainty, but it seems that the trunk-cavities are .separated by a ventral 

 mesentery, as in fig. 197. 



Fig. 195 appears at first sight to indicate that the two collar-cavities become continuous 

 with one another in the middle dorsal line. The sections are, however, not accurately horizontal, 

 and an e.xamination of the neighbouring sections leads to the result that the cavity seen in 

 fig- 195 is really the dorsal end of the right collar-cavity. 



Figs. 191 — 194 illustrate the structure of an advanced embryo as seen in transverse 

 sections. F"ig. 191 shews the anterior sense-organ (i'. <?.), on either side of which is a mass of 

 what seems to be nerve-tissue («. /.). The dorsal two thirds of the ectoderm is composed, as 

 is usual in embryos at this stage, of vacuolated cells. In the ne.xt section further back (not 

 drawn) the nerve-tissue can be seen underlying the ventral ectoderm, while in later sections 

 (figs. 192 — 194) it underlies the greater part of the ectoderm, wherever this layer has assumed 

 the form characterised by the absence of conspicuous vacuoles. There is in fact, in this embryo, 

 a general epidermic nerve-plexus similar to that of Balanoglossus. There appears to be a 

 slight concentration of this plexus at the angle between the wall of the ventral invagination 

 and the lateral ectoderm (fig. 192). A ventral nerve is perhaps visible in fig. 193. 



The ventral invagination consists partly of vacuolated cells, intermingled with the narrow 



i) It might be possible to make a detailed comparison between the embryo of C. gracilis and that of one of the Ectoprocta, 

 based on the similarity of the internal mass of undifferentiated yolk and on the pyriform organs of the two embryos, and comparing 

 the ventral invagination of Ccphalodiscus with the "internal sac" of the Ectoprocta. Believing as I do that the Ectoprocta are related to 

 the Entoprocta, and that the larvae of the latter indicate afTinitics with other groups characterised by the possession of a Trochosphere 

 larva, I am not at present prepared to admit that the embryology of CcphaloJiscus suggests any affmity between that animal and 

 the Polyzoa. 



