vii MOLLUSCA — THE SHELL 61 



shell, for the closing of which there may even be an operculum, secreted by the foot, 

 as in the Prosobnnichiti. 



The following are some of the principal stages and concomitant phenomena of 

 the reduction of the shell : (a) The well-developed shell ceases to be large enough to 

 shelter the whole body, (b) The shell, which becomes thinner and smaller, is dorsally 

 overgi-own, partially or altogether, by extensions of the mantle, (c) As the shell 

 (which is then either cup- shield- or ear-shaped) becomes continually smaller, the 

 visceral dome begins to be levelled down, till it no longer rises above the rest of the 

 body, its contents spreading out to a certain extent over the dorsal surface of the foot. 

 (d) The external asymmetry of the body passes by degrees into symmetry, whereas 

 the internal asymmetry never entirely disappears, (c) The shell is reduced to a 

 number of isolated calcareous particles in the integument of the flattened visceral 

 dome. (/) There is at last no trace of a distinct visceral dome ; calcareous particles 

 are to be found in the dorsal integument of the long and now naked Gastropod. 

 ((/) Even these particles finally disappear. 



In connection with the reduction of the shell in Opisthohrcmchia and Pulmonata, 

 compare the section on the mantle, pp. 43-48. 



The Heteropoda present the following interesting series : — 



Atlanta. The shell is very light and thin, but large and spirally coiled (with 

 an incision at its aperture) ; the animal can entirely withdraw into it, and close 

 it by means of an operculum developed on the distinct metapodium. 



Carinaria. The shell is thin, light, and delicate ; it is cup-shaped, and covers the 

 large stalked visceral dome, but is incapable of sheltering the long and thick 

 cylindrical body and foot. There is no operculum. 



Pterotrachea. The visceral dome is small, and there is no shell and no operculum. 



C. Lamellibranchia. 



The two lateral valves of the Lamellibranch shell are connected, at their 

 dorsal edges, by means of a hinge and a ligament. The ligament counteracts the 

 muscles of the shell, which will be described later on, and which, by their contraction, 

 close the shell. It is usually composed of two layers, the inner layer being elastic, 

 while the outer is not. The outer non-elastic layer passes into the epidermis or 

 periostracum of the shell. The inner layer of the ligament is elastic and calcareous, 

 and is often called cartilage, but this is histologically incorrect. 



The ligament lies either externally, distinctly seen dorsally between the 

 prominences of the hinge edges of the valves, or internally, stretched between the 

 apposed edges of the hinge, which are furnished with depressions for its reception. 

 These depressions can easily be distinguished from those belonging to the hinge 

 itself, since the former are alike on the two valves, whereas the furrows and other 

 depressions belonging to the one face of the hinge correspond with teeth, ridges, 

 etc., on the opposite face. 



When the elastic "cartilage" of the ligament is at rest, as in a dead bivalve, or 

 when the adductor muscles of the living animal are relaxed, the valves open. W]ien 

 the adductors contract, the "cartilage" is— ajiparently in all cases— compressed. 

 On the other hand, when the adductors are relaxed, the elasticity of the ' ' cartilage " 

 forces the shell open again (Fig. 61). 



The continuity established between the two valves, by means of this dorsal 

 ligament, causes the Lamellibranch shell to appear to consist, strictly speaking, of 

 one dorsal piece, developed to the right and left ventrally into two valves. The 

 constitution of the ligament and hinge are of importance in classification. 



We must refer the reader to systematic zoological works for the special forms 

 taken by the shell, and content ourselves with the following remarks :— 



