ON THE INVAGINATE PLANULA. 161 
yet joined or broken into that portion of the alimentary 
canal formed by invagination from the anal pole. In the 
central dilatation of the invaginated alimentary sac granular 
matter and corpuscles are seen which attain subsequently a 
considerable development. 
In the present communication I content myself with 
drawing attention to the facts afforded by Paludina confir- 
matory of those views as to the early development of the 
Mollusca which I have put forward in my memoir commu- 
nicated to the Royal Society in February, 1874, as well as in 
a memoir on Limneeus published in October, 1874 (* Quart. 
Journ. Microsc. Science’), and which also are indicated in 
shorter notices in my papers in the ‘ Annals of Nat. Hist.,’ 
1873 (February and May). The main point is this—that 
in the Mollusca the endoderm or hypoblast originates by 
an invagination,’ which, if the invaginated mass be large 
and contain food-granules and remain little or not at all 
broken up into cells, may be spoken of as a case of over- 
growth or epibolé (Selenka). In this case the invaginated 
or overgrown mass gives rise to the endoderm or hypoblast 
by a segregation and cleavage subsequent to the invagina- 
tion. If the invaginate mass contain a small amount of 
food-granules it will break up into cells before invagination, 
and thus form at once an alimentary sac by embolé 
(Selenka). : 
So far my observations and views with regard to Mollusca 
are but confirmatory of those of Kowalewsky relative to 
Euaxes and Lumbricus, and of Selenka (see his paper on 
« Purpura,” ‘ Niederlandische Archiv,’ vol.i, 1872). But I 
have established the following important point with regard to 
those diploblastic Planule among molluscs which arise by 
embolé, viz. that the orifice of invagination either closes 
(Pisidium, Limneus, Nudibranchs) or remains open as 
anus (Paludina). Possibly even in Paludina it closes and 
reopens after a short interval. Further, the neck of the 
‘ In all animals this ¢zvagination of the primitive polyplast or mulberry 
sphere takes place sometimes in a more, sometimes in a less obvious man- 
ner, excepting in the Hydrozoa. In the Hydrozoa the endodermal layer 
forms by delamination, that is to say, it is formed by cell-division on the 
inner surface of a hollow sphere formed by a single layer of cells. Hence 
Planule are primarily divisible into delaminate and invaginate. They appear 
to arise by two varieties of the same process, differing chiefly as to the ¢ime 
at which the separation of the endodermal and ectodermal constituents 
takes place. From a comparison of the different conditions of the Planula 
presented by vertebrate animals, my friend Mr. Balfour has arrived at the 
conclusion that the Mammalian diploblastic phase arises by a modification 
of the process of invagination, 
