CHAPTER IX. 



MOLLUSCA 1 . 



ALTHOUGH the majority of important developmental features 

 are common to the whole of the Mollusca, yet at the same time 

 many of the subdivisions have well-marked larval types of their 

 own. It will for this reason be convenient in considering the 

 larval characters to deal successively with the different sub- 

 divisions, but to take the whole group at once in considering the 

 development of the organs. 



Formation of the layers and larval characters. 

 ODONTOPHORA. 



Gasteropoda and Pteropoda. There is a very close agree- 

 ment amongst the Gasteropoda and Pteropoda in the general 

 characters of the larva ; but owing to the fact that the eggs of 



1 The classification of the Mollusca adopted in the present chapter is shewn in the 

 subjoined table : 



I. ODONTOPHORA. II. LAMELLIBRANCHIATA. 



1. Gasteropoda. . Dimya. 



a. Prosobranchiata. b. Monomya. 



b. Opisthobranchiata. 



c. Pulmonata. 



d. Heteropoda. 



2. Pteropoda. 



a. Gymnosomata. 



b. Thecosomata. 



3. Cephalopoda. 



a. Tetrabranchiata. 



b. Dibranchiata. 



4- Polyplacophora. 



5- Scaphopoda. 



B, II. 15 



