I I : 



If it be permitted to appeal to animals which though less nearly related to Cephalodiscus 

 are still, as it appears to me, possessed of affinities to Balanoglossus, allusion may be made 

 to Amphioxus and Echinoderms, in support of the explanation given above of the structure 

 of the embryo of Cephalodiscus. The anterior body-cavity of Amphioxus, as described by 

 Hatschek, is developed from the anterior end of the original archenteron, and shortly before 

 its separation from the mesenteron it forms a cavity projecting backwards beyond the anterior 

 end of the mesenteron in much the same manner as that in which the anterior yolk-mass of 

 Cephalodiscus overlaps the remainder of the yolk. A similar arrangement has been described 

 in many Echinoderms, as by Ludwig and MacBride in Asterina, by Masterman ^) in Cribrclla, 

 and by MacBride °) in Echinns. 



The characteristic relation of the collar-cavities in the embryo of Cephalodiscus to the 

 constricted region of the yolk makes it by no means improbable that these cavities are true 

 enterocoels, given off from the archenteron in this region ; while fig. 1 94 appears to suggest 

 that the third body-cavities have a definite relation to the posterior end of the yolk-mass. It 

 is not impossible that the somatic epithelium of the paired body-cavities is first differentiated 

 at the surface of the yolk, and that the splanchnic epithelium gradually spreads round the yolk- 

 mass in the manner which might be indicated by the posterior end of fig. 201. Against this 

 view may be set the fact that the paired coelomic spaces, at the earliest stage at which they 

 have been recognised, appear to be separated from the yolk by a distinct membrane (figs. 208, 

 209), even though no epithelial lining can be detected on the outer side of this membrane. 



The general arrangement of the coelomic spaces in the embryo appears to me to have 

 a close resemblance to that shewn by Bateson in his well known diagram (84, PI. XXI, 

 fig. 40) of the embryo of Dolichoglossjis kowalevskii, with the exception of the fact that the 

 archenteron is represented, in Cephalodiscus., by a practically solid mass of cells. 



XVIII. AFFINITIES. 



In 1887 I had the opportunity of bringing forward evidence tending to shew that Cephalo- 

 discus must be placecl in the same group with Balanoglossus. I indicated at the same time my 

 belief that it might have affinities with Phoronis, though not with the Polyzoa. It appeared to 

 me not improbable that RhabdopleJira was related to Cephalodiscus^ as had been assumed from 

 the first by M'Intosh, Lankester and others-, but in view of the want of evidence with regard 

 to the structure of Rhabdoplcura I refrained from expressing a definite opinion on this part 

 of the question. 



The subject of the affinities of Cephalodisctis has aroused a good deal of interest in 

 recent years. It may conveniently be considered under several headings, as follows: — namely 

 the affinity of Cephalodiscus to; 



1) Trans. Roy. Soc. Edinburgh, XL, Pt. 2, 1902, p. 3S5 f. 



2) Phil. Trans. Roy. Soc. Londen (B), CXCV, 1903, p. 296. 



