MOLLUSCS. 213 



and a fleshy mantle fold on either side, enclosing a pair of 

 branchial chambers between mantle and foot. 



3. The mantle usually secretes a shell which increases 

 in size during life, the stages of increase being shown by 

 lines of growth. 



4. The true branchiae project into the branchial chamber; 

 they may be lost and replaced by other gills developed 

 from other parts of the body. 



5. A characteristic feature of all except the Acephala 

 is the lingual ribbon. 



6. The nervous system consists of at least three pairs 

 of ganglia connected by nervous cords. 



7. The reproduction is exclusively by eggs. A trocho- 

 phore may occur in development. 



8. The Mollusca are divided into Amphineura, Gastero- 

 poda, Scaphopoda, Acephala, and Cephalopoda. 



9. The AMPHINEURA have a very simple nervous sys- 

 tem. In the Chitons, which belong here, there are eight 

 plates of shell on the back. 



10. The GASTEROPODA have a distinct head with ten- 

 tacles and a creeping foot. 



11. The shell is unpaired, more or less conical, and fre- 

 quently spirally coiled. It is occasionally lacking in the 

 adult. 



12. The Gasteropoda include the Diotocardia, Mono- 

 tocardia, Opisthobranchia, and Pulmonata. 



13. The SCAPHOPODA are primitive forms with tubular 

 shell and no distinct head. 



14. The ACEPHALA lack head and lingual ribbon, and 

 have a bivalve shell. 



15. The edges of the two mantles may unite behind to 

 form the siphon. 



16. The foot is fleshy and more or less hatchet-shaped. 



