Miscellaneous. 91 



cause for tho genetic connexion of the vaiious branches of the 

 uninml kingdom. We must meet the direct issue raised by 

 Hackel (that such a genetic connexion either does or does not 

 exist) by repeating what has so often been said by others : — This 

 genetic connexion may exist; but we have at present no proof that 

 it does exist. And, at any rate, his Gastrata theory does not bring 

 us any nearer to a mechanical exjilanation of such a genetic con- 

 nexion, however probable it may be 



Here we must call attention to a marked difference between 

 Acalephs and Polyps on one side, and Echinoderms on the other — 

 that while in tlie former the connexion between the digestive 

 cavity and the water-system always remains open, it is at one time 

 disconnected in the Echinoderms, though it is eventually reopened 

 through anastomoses of the water-tubes. The anal opening holds 

 in Ctenophora very much the same relation which it holds in 

 Echinoderm larvaj, in which the water-tubes are still connected 

 with the primitive digestive cavity. "When we find, as we do, 

 that in Ctenophora, as well as in Echinoderms, the primitive diges- 

 tive cavity is formed by the inturning of the ectodcnn, that in 

 both classes the water-system is developed as diverticula from 

 this digestive cavity, we fail to see how we can separate the Cteno- 

 phora from Echinoderms and place them with Polyps in a separate 

 subkingdom of the animal kingdom. Xo one questions the rela- 

 tionship of Ctenophora to Acalephs ; yet from embryological data it 

 would be more natural to associate Echinoderms and Ctenophora 

 into one subkingdom, characterized by the mode of formation of 

 the water-system as diverticula forming eventually chymiferous 

 tubes in both classes, and to associate the other Acalephs with the 

 Polyps*, where the chymiferous tubes and cavities are formed by 

 the liquefaction of the interior of the Planula. Any one who will 

 compare the figures of the embryos of starfishes (A. Agassiz, Em- 

 bryol. Starfish, pi. ii. fig. 8) and Ctenophora (pi. iii. figs. 6-10, 

 pi. V. figs. 5, 11) at the time when the chymiferous tubes are 

 reduced to mere diverticula, cannot fail to feel satisfied of their 

 complete identity of plan. Metschnikotf has made, in addition to 

 the homologies I have just recalled, a most interesting comparison 

 between an Echinodemi larva and a Ctenophore ; he shows that, 

 even in the adult Ctenophore, the identity of plan is not destroyed, 

 and is carried out to the smallest details.. The only point in which 

 I would differ from him is in his comparison of the abactinal coeliac 

 openings to the actinostome : he seems to forget that in Echino- 

 derm larvae what at first performed the part of anus and mouth 

 eventually becomes the mouth alone ; so that his figures should be 

 reversed, and then the identity will be found complete between an 

 Echinoderm larva (see A. Agassiz, Embryol. ^Starfish, pi. iii. 

 fig. 0, and pi. vii. fig. 8) with its oesophagus, digestive cavity, ali- 

 mentar}' canal and its chymiferous pouch (water-system), from 

 which run the diverticida eventually to become the water- tubes, 



* See AUman's views on the position of the Ctenophora as contr.;sted 

 with the Actinozoa, Trans. K. S. Kdinb. xxvi. pt. ii. p. 46(3, 1871. 



