I I 2 



If it lie permitted to appeal to animals whicii thoui,r]i less nearly related to Cephalodisais 

 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 giver) above of the structure 

 of the embryo of CepJialodisais. The anterior body-cavit\- 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 

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

 in many Echinoderms, as by Ludwh, and MacBride in Astei'ina, by Masterman ^) in Cridrclla, 

 and by MacBride ") in Ec/iinus. 



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

 constricted region of the yolk makes it b\- no means improbable that these cavities are true 

 enterocoels, given off from the archenteron in this region; while fig. 194 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 llie splanchnic epithelium gradually spreads round the \olk- 

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

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

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

 209), even though no epithelial Iming 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 B.\teson in his well known diagram (84, PI. XXI, 

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

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



XVIII. AFFINITIES. 



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

 discus must be placed in the same group with Balanoglossus. 1 indicated at the .same time mv 

 belief that it might have affinities with PJioronis^ though not with the Polyzoa. It appeared to 

 me not improbable that Rhabdopleiira was related to Ccphalodiscns, 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 Rhabdoplcnra I refrained from expressing a definite opinion on this part 

 of the question. 



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

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

 the affinity of Ccphalodiscns to: 



i) Trans. Roy. Soc. Edinburgh, XL, I't. 2, 1902, p. 385 f. 

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



