THE SHELL. 2Y 



small size compared to the enormously large aperture, which 

 hugs close to rocks, etc., to which the animal attaches itself by 

 its much expanded sole almost as tightly as though it were a 

 specialized sucker. The branchial cavity is placed in communi- 

 cation with the surrounding water in these genera by a special 

 notch in Haliotis dsxxiii, 10-13), which, by the progressive 

 growth of the shell forms a series of dorsal apertures ; the earlier 

 ones finally becoming filled again with shell, but several always 

 remaining open. In Fissurella (Ixxxiii, 15), a similar but single 

 aperture exists at the apex of the shell, and in several allied 

 genera a similar provision is made only difiering in form and 

 position. 



A large volume might readily be filled with interesting partic- 

 ulars concerning the shells of mollusca, but our space will only 

 allow the above rapid resume of the more important aspects of 

 the subject. It remains to detail briefly the special relations of 

 the shell to each class of mollusks, together with the nomen- 

 clature which has been adopted for the ready classification of 

 its diverse forms. 



Cephalopoda. An internal shell is found only in the dibran- 

 chiate cephalopods, and amongst them only fully developed in 

 the decapod division, %. e., among the pelagic species, whose 

 quick movements require the support to the body which the 

 form and position of the shell aflTords ; nevertheless it is prefig- 

 ured in the cartilaginous blades lodged in the back of the 

 octopods (iv, 56), and is more fully developed in the single 

 octopod genus, Cirroteuthis (iv, 58), which possesses the means 

 of more rapid motion in its largel}^ webbed arms. 



The internal shell is simple usuallj'', in form, being a blade or 

 pen lodged in a pouch or slit in the back of the mantle, with an 

 anterior, more or less specialized, prolongation of its rhachis or 

 quill. The internal shell is either horny or chitinous and 

 transparent as in Loligo (xxv, 21), or a spongy, chalky mass, as 

 in Sepia (xxvii, 49), or calcareous, as in the fossil Belemnites 

 (ii, 19, 21), or mother-of-pearl, as in Spirula (xxvii, 52) ; in the 

 latter, only, taking the spiral form of the external shells of the 

 tetrabranchiate cephalopods. 



The feather-shaped horny shell of the Loligo resembles, and 

 is called, a. pen, and its rhachis, prolonged in front like a quill, 

 completes the resemblance. This rhachis is on the ventral side, 

 when the j^en is lodged in the mantle, the wing end being 

 posterior, the quill anterior. The wings, or lateral projections 

 are, commonly, broader in the female than in the male individuals. 



When the shell is both corneous and testaceous, as in Sepia, 

 among recent genera and in several fossil forms, it consists of a 

 thin, horny blade, laying beneath the entire dorsal surface of the 



