On the Developmental History of the Mollusca. 85 



harmonized with, the formation of Gastrula! by the embolic or epi- 

 bolic forms of invagination. 



It would certainly seem at present that the orifice of invagi- 

 nation of the invaginate Gastrula must not be regarded as the 

 equivalent of the later erupting mouth' 'of the segregate Gastrula*, 

 which is the true permanent mouth of the Sponge or Coelenterate. 

 In no case is the orifice of invagination of the invaginate Gastrula 

 known to persist under any form ; it appears solely to effect 

 the invagination, and when that is effected vanishes. 



Enough has been said to show the importance of observations 

 relating to the Gastrula-\)hnse of development. In the paper 

 well-marked invaginate Gastrula} are described from : — 



1. Pisidium (Lamellibranch). 



'2. Tergipes (Nudibranch). 



3. Polycera (Nudibranch). 



4. Lhnax (Pulmonate). 



5. Limnceus (Pulmonate). 



In addition to these cases of the development of invaginate 

 Gastrula! among Mollusca, the examination of the very beautiful 

 figures in the papers of Love'n on molluscan development leaves 

 no doubt that he has observed invaginate Gastrula; in the follow- 

 ing cases, but has not understood their structure : — 



6. Cardium (Lamellibranch). 



7. Grenella (Lamellibranch). 



Similarly, Karl Vogt's observations on Action indicate the same 

 state of things as the author has pointed out in Polycera ; and 

 hence we may add : — 



8. Aetceon (Nudibranch), 



and, finally, from Kowalevsky's statement, though not accom- 

 panied by figure or description, 



9. Atalanta (Heteropod). 



The second matter of theoretical interest (namely, the early 

 features in the development of the shell) has not been previously 

 discussed, since the structures described in the paper as shell- 

 patch, shell-groove, and shell-plug were unknown. 



If, as seems justifiable, the Cephalopoda are to be regarded as 

 more nearly representing the molluscan type than do the other 

 classes, or, in other words, more closely resemble the ancestral 

 forms than they do, we might look, in the course of the develop- 

 ment of the less typical Mollusca, for some indication of a repre- 

 sentative of the internal pen of the higher Cephalopoda. We might 

 expect to find some indication of the connexion between this and 

 the calcareous shell of other forms ; in fact the original shell of all 

 Mollusca should be an internal one, or bear indications of a possible 

 development into that condition. 



In Pisidium, in Aplysia, and in Neritina the author has submitted 

 evidence of the existence of a specially differentiated patch of 



* In his paper in the 'Annals' for May 1873 the author has inclined to 

 the view that it may be so regarded. 



