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 t/ie 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 : 



II. LAMELLIBRANCHIATA. 

 a. Dimya. 



I. OUONTOPHORA. 

 i- Gasteropoda. 



a. Prosobranchiata. 



b. Opisthobranchiata. 

 . Pulmonata. 



d. Heteropoda. 



Pteropoda. 



a. Gymnosomata. 

 f>. Thecosomata. 



3- Cephalopoda. 



a. Tetrabranchiata. 

 t>. Dibranchiata. 



4- Polyplacophora. 



5- Scaphopoda. 



r, II. 



/'. Monomya. 



