160 INSECTS MOLLUSCA DIAGRAM /. 



SUB-KINGDOM MOLLUSCA. 



This sub-kingdom has been divided into several groups, of which 

 the most important are the Cephalopoda, which have tentacles 

 which they use for feet (cuttle-fish) ; the Gasteropoda, which crawl 

 on their belly (slugs, snails) ; the Acephala, without heads, en- 

 closed in bivalve shells (oyster, mussel). 



The animals of this class have neither vertebra, like the verte- 

 brata, nor segments like the articulata. Their skin is smooth, as 

 in slugs ; but they are sometimes enclosed like snails and oysters 

 in a shell varying in hardness. Molluscs generally breathe by 

 means of branchiae ; but some among them, as the slug for in- 

 stance, breathe by a lung which opens on the side, behind the 

 head. 



The slug is a mollusc which will give the best idea of the animals 

 of its class. It crawls about in damp weather or in the evening, 

 and moves by dragging itself on what is called its foot. Its whole 

 bodyris covered with a viscous mucous which leaves a trail behind 

 it. It devours plants and fruits, and does great injury in the 

 garden. * 



The snail is very different from the slug. It walks like it by 

 creeping on its foot ; but it has a shell into which it retires, when 

 danger threatens, or to pass the winter. The snail's shell thus 

 serves it for a house ; but it forms part of itself ; it is attached to 

 its skin, and it cannot go out of it, as is sometimes supposed. 



