Guide to the Invertelrata. 



ffoUusca. secretes the shell, and is named the mantle. The shell may he 

 tAXLEEIES univalve, or in one piece, as in the garden-snail, the whelk, etc. : 



Fig. 3. — Bivalve shell of Glossus [Isocardia) cor, Linn. 



or it may be bivalve, composed of two parts, as in the oyster, 

 the mussel, the cockle, etc. ; or multivalve, composed of as 

 many as eight pieces, as in the shell of the Chiton. In the 



Fig. 4. — Multivalve shell of Chiton squamosus, Linn. 



bivalves the mantle is divided into two lobes; in the univalves 

 it takes the form of a sac, with an opening in front. 



Fig. 5. — Melix desertorum, Forsk., the desert- snail. Egypt. 



