75 
while there is in many eases a true gastrula stage, its orifice 
bears no constant relation to the definitive mouth and anus.. 
There does not seem to be any valid reason for disputing 
Fol’s statement (Etudes sur le developpement des Mollusques ? . 
I. Pteropodes, arch. d. Zool. exp. et gen. IY, 1875), that on 
the Pteropods the orifice of invagination persists and becomes, 
the definitive mouth. We have the testimony of both Lan- 
kester and Butschli • (“ On the Invaginate Planula or Diplo- 
blastic phase of Paludina vivipara,” by E. Pay Lankester. 
Quart. Jour. Mic. Sc. XY, 1875; and “On the Coincidence 
of the Blastopore and Anus in Paludina vivipara,” by E. Pay 
Lankester. Quart. Journ. Mic. Sc. XYI, 1876, and “ Ent- 
wickelungsgeschichtliche Beitrage. I. Jur Entwickelungsges- 
chichte von Paludina vivipara, von O. Butschli, zeitsche. f. 
Wiss. Zool, XXIX), to prove that in Paludina the orifice of" 
invagination persists and becomes the anus. 
In the oyster the orifice of invagination closes up, and forms 
the shell and in the Squid it is perfectly certain that the point 
where the blastoderm folds in over the food-yolk has no con- 
nection with either the mouth, the anus or the shell-gland. 
It is not necessary to extend the above list by references to 
observers who have given still different accounts. There are 
fine grounds for disputing the correctness of many of these 
papers, but I do not think the present state of our knowledge 
gives us any reason for doubting the conclusions of Fol, Lan- 
kester and Butschli ; my own observations on the oyster are 
confirmed by Pabl, and in a future paper I hope to show that 
the Squid is such a favorable subject for study that there is 
no chance for error in the statement which I have given above^ 
and we may conclude that the present condition of embry- 
ology fully justifies Lankester’s statement that in the Mol- 
lusca there is no necessary connection between the blastopore 
and either the mouth or the anus. 
Xow there cannot be the least doubt that the molluscan 
mouth is the same opening in all the classes, and that the 
anus is also homologous throughout the whole group. 
