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 subdivisions, but to take the 

 whole group at once in considering the development of the organs. 



Formation of tlie layers and larval characters. 

 ODONTOPHORA. 



Gasteropoda and Pteropoda. There is a very close agreement 

 amongst the Gasteropoda and Pteropoda in the general characters of 

 the larva; but owing to the fact that the eggs of the various species 

 differ immensely as to the amount of food-yolk, considerable differ- 

 ences obtain in the mode of formation of the layers and of the 

 alimentary tract. 



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

 subjoined table: 



I, ODONTOPHORA. II. LAMELLIBEANCHIATA. 



1. Gasteropoda. a. Dhnya. 



a. Prosobranchiata. b. Monomya. 



b. Opisthobranchiata. 



c. Pulmonata. 



d. Heteropoda. 



2. Pteropoda. 



a. Gymnosomata. 

 6. Thecosomata. 



3. Cephalopoda. 



. Tetrabranchiata. 

 6. Dibranchiata. 



4. Polyplacophora. 



5. Scaphopoda. 



