Miscellaneous. 91 
cause for the genetic connexion of the various brauches of the 
animal kingdom. We must meet the direct issue raised by 
Hickel (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 Gastrwa theory does not bring 
us any nearer to a mechanical explanation 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 the 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 larve, 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 ectoderm, 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. No 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, pl. 11. fig. 8) and Ctenophora (pl. i. figs. 6-10, 
pl. 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. Metschnikoff has made, in addition to 
the homologies I have just recalled, a most interesting comparison 
between an Echinoderm 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 
J would differ from him is in his comparison of the abactinal cceliae 
openings to the actinostome: he seems to forget that in [Echino- 
derm larve 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, pl. iii. 
fig. 6, and pl. vu. fig. 8) with its cesophagus, digestive cavity, ali- 
mentary canal and its chymiferous pouch (water-system), from 
which run the diverticula eventually to become the water-tubes, 
* See Allman’s views on the eoeue of the Ctenophora as contrasted 
with the Actinozoa, Trans. R. 8S. Edinb. xxvi. pt. 1. p. 466, 1871. 
