Geology and Natural History. 475 
“What Heckel substitutes in the place of the accepted types of 
the animal kingdom is simply another view of these same types, 
and nis Gastrea theory is in no danger of upsetting, at present at 
least, zodlogical classification as now understood. Indeed, if we 
cell? ‘There we have a definite starting point, a typical ae 
which underlies the whole of the animal ‘king om and which for 
the walls of Heckel’s gastrula. Then we shall all be agreed, sid 
when we frankly state that all organisms are eS — a prim- 
itive cell and from its subsequent increase come within the 
ange of positive knowledge, but we are sirireuuabaly as far as 
ever from having for that reason been able to trace a mechanical 
cause for the genetic pageemeeg: of the various branches of the 
animal kingdom meet the direct issue raised by 
eckel,—that such a eanetis connection either does or does not 
exist —by repeating what has so often been said by others, ree 
genetic connection may exist, but we have at present no proof t 
it does exist, and at any rate his gastrea theory does not seeing a 
any nearer to a cages explanation of aun . genetic ee 
tion however probable it 
“ Here we must call stienuin to a ee enue sede 
m Echinoderms and place them awk Polyps in a — 
subkingdom of the animal kingdom. No one questions the relati 
ship of Ctenophore to Acalephs, yet from embryological data it 
would be more natnral to associate Echinoderms and Ctenophore 
into one subkingdom characterized by the mode of formation of 
the water system as diverticula, forming svecitually 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 poco Any one who will 
compare the figures of the embryos of starfishes (A. Agassiz, ord 
bryol. Starfish, pl. 1, fig. 8) and Ctenophore tan m, figs. 6-10 
pl. v, figs. 5, 11) at t the time _— the chymifero s tubes are re- 
duced to mere diverticula, cannot fail to feel satisfied of their 
complete identity of plan. Metschnikoff has made, in addition to 
hinblogies T have just recalled, a most interesting comparison 
* See Allman’s views on the position of the Ctenophore as contrasted to the 
Actinozoa. Trans. R. S. Edinb., xxvi, pl. m1, p. 466, 1871. 
- 
