ON THE INVAGINATE PLANULA, 165 
the blastopore. ‘The blastopore rapidly narrows and 
closes up. The anus subsequently appears as a new per- 
foration at the point corresponding to the cicatrix of the 
blastopore. In some cases it is possible that the blastopore 
persists as the permanent anus. It is not proved in any 
animal to persist in any other relation. 
=FEmsotic INVAGINATE PLANULA. 
These have been observed in the case of Amphioxus, 
Ascidians, many Molluscs, Sagitta, Echinoderms, and many 
Worms. 
2. Or from the first commencement of the post-seminate 
development of the egg there is a relatively very much 
larger amount of food-material incorporated with the endo- 
dermal portion than with the ectodermal portion of the 
segmenting polyplast, so that the latter may be spoken of 
as the “segregate” or “active yelk,” the former as the 
“residuum ’”’ or “residual yelk.” The residual yelk (endo- 
dermal portion of the polyplast) generally is but little or not 
at all segmented, and is overgrown or enveloped by the 
advancing cells of the active segregate yelk (ectodermal por- 
tion of the polyplast). The blastopore closes early, and 
has never been supposed to persist either as anus or as 
mouth! The subsequently developing anus sometimes cor- 
responds approximately in position with the cicatrix of the 
blastopore. 
=Epipoitic INvAGINATE PLANUL2. 
These have been observed in the case of many Molluscs, 
many Vertebrates, the Ctenophora, and certain Worms, and 
certain Arthropods. 
In coaclusion, I may touch on another matter to which 
attention is drawn by Metschnikow in an article in ‘ KOll. 
und. Sieb. Zeitschrift’ for 1874, and by Professor Huxley in 
an article on the classification of animals (this Journal, Jan., 
1875). In Echinoderms and in Sagitta—most clearly in 
the latter—the body cavity commences as a diverticulum 
of the alimentary canal, in fact, as a gastro-vascular space, 
comparable to that space in Celenterata. Atthe same time 
Schulze (F. E.) has described in Sarsia the formation of a 
space in the gelatinous tissue of the disc by the simple split- 
ting of the deep layers of the tissue. You have in the latter 
an undeniably “ schizocelous” condition, in the former an 
“ entero-ceelous’’ condition, to use Professor Huxley’s termi- 
nology. I wish now very briefly to point out that, viewing the 
1 Except by Selenka, who considers that it becomes the mouth in Purpura. 
