FIG. S3. MOLLUSCA. 



1, Nautilus (Argonauta); 2, Clio Borealis ; 3, Mussel (Mytilus edule). 



The last of the three great divisions of the Invert ebrata 

 is formed by the Mollusca ; it is divided into six classes, 

 (see page 218). The Mollusca are characterised by having 

 no internal skeleton, nor external horny case, as in the 

 Articulata ; they are rather soft, and either void of solid 

 covering, or possessed of "shells" composed chiefly of earthy 

 matter (chalk), and in one or two pieces, called valves, 

 hence the names, uni- valve and bi-valve ; all shells of this 

 description belong to the Mollusca, some of which have no 

 means of locomotion, as Oysters ; others have a "foot" 

 covered with a muscular expansion, called the "mantle," 

 xby which they glide onwards, as the Snail. 



1. The Tunicata have no shell or hard covering, but 

 are of a pretty firm consistence ; they are either 

 fixed to rocks and sea-weeds, or float about freely, 

 and are either solitary, social, or compound. The " As- 

 cidians " are united in groups, and are all connected 



