1874.] Developmental History of the Mollusca. 233 



9. The disappearance of the primitive mouth, and the development of a 

 secondary mouth. 



10. The development of a pair of large nerve-ganglia by invagination 

 of the epiblast immediately below the primitive eye-chambers. 



General Considerations relative to the Observations contained in Sections II., 

 III., IV. (containing the developmental histories of Pisidium, Aplysia, 

 Tergipes, Polycera, and Neritina). 



In these observations the author points out briefly their bearing on two 

 matters of theoretical importance, viz. (1) the origin and significance of 

 what has been called the Grastrula-ph&se of development, and (2) the ho- 

 rn ologies or homogenies (as the author prefers to say) of the shells, liga- 

 ments, and internal pens of the Mollusca. More facts have to be sought 

 out and brought to bear on these questions ; but the author, while occu- 

 pied in that further search, indicates the anticipations which must guide 

 and stimulate it. Before doing so he mentions that there are a variety 

 of other matters of interest in the facts recorded in the paper which can- 

 not yet be brought into any theoretical structure, but which are not on 

 that account kept back, as they will probably be of some service in their 

 isolated condition. 



Kowalevsky was the first to describe, in a precise manner, the forma- 

 tion of the foundations of the alimentary tract in a developing embryo, 

 by invagination of the wall of a simple primitive blastosphere, or hollow 

 ball of embryonic cleavage-corpuscles. He detected this mode of deve- 

 lopment in Amphioxus, and subsequently in Ascidia. By later researches 

 he was able to indicate the same mode of development in certain Vermes 

 (Sagitta, Euaxes, Lumbricus) ; and he mentioned incidentally that he had 

 observed a similar development in the Heteropodous mollusk Atalanta. 

 At that time the author was studying the development of Pisidium and 

 Limax, and obtained evidence of the invagination of the primitive blasto- 

 sphere in those two widely separated mollusks. Subsequently at Naples 

 he found the same process occurring in Nudibranchs. The probable 

 identity of this process of invagination with that so well known in the 

 Batrachians, especially through Strieker's admirable work on the subject, 

 became clear, to those occupied with embryological studies, from the facts 

 established by Kowalevsky ; and the " anus of Eusconi " could now be re- 

 cognized in the " orifice of invagination " present in members of the three 

 large groups of Vermes, Mollusca, and Vertebrata. 



The embryonic form produced by this invagination-process is a simple 

 sac composed of an ectoderm and endoderm, with an orifice connecting 

 the exterior with the cavity lined by the endoderm. It, in short, pre- 

 sents the typical structure of the simplest Crelenterata, and corresponds 

 exactly with the so-called Planula of the polyps and corals. Hence we 

 are tempted to see in this primitive invagination-form the representative 

 of the Coelenterate phase of development of the whole animal kingdom. 



