330 CHARACTERS OF INVERTEBRATE ANIMALS 



Structure and Formation of the Shell. The shell is a horny 

 structure largely impregnated by salts of lime. It is produced 

 by the underlying epidermis, from which a sticky substance 

 exudes which afterwards becomes hard. In all classes of Mol- 

 luscs the mantle has a great deal to do with the formation of 

 the shell, but even here, where the mantle is very well de- 

 veloped, it does not line the part of the shell near the hinge- 

 line, and yet that part can be repaired if broken. In a Garden 

 Snail the mantle is of comparatively small extent, the chief part 

 of it being the roof of the lung, yet all parts of the snail's 

 shell can be repaired. The shell of tfte Mussel, and the same 

 thing is true for a land- or sea-snail, consists of three layers: (i) 

 a greenish external skin, often called incorrectly the epidermis; 

 (2) a middle prismatic layer in which the calcareous material 

 consists of oblique prisms; and (3) an internal pearly layer, 

 made up of numerous thin lamellae, the edges of which form a 

 series of minute wavy ridges on the inner side of the shell, which 

 are the agents to which the rainbow tints of the pearly layer are 

 due. The edge of the mantle is thickened, and, both here and 

 in most Mollusca, plays a very important part in the formation of 

 the shell, the two outer layers originating from it alone. Repairs, 

 therefore, except at the edge of the shell, are carried out in 

 pearly material only, though further observations are wanted on 

 the subject. 



The two valves are united together in the region behind the 

 umbo by a horny elastic band known as the ligament, which is 

 kept on the stretch when the shell is closed. In the shell of Unio 

 (fig. 191) there are projecting teeth along the inner side of the 

 hinge-line in each valve, which fit into corresponding sockets in 

 the other valve. Such teeth and sockets are entirely absent in 

 Anodon, whence its name (Gk. an, without; odous, a tooth). The 

 inner side presents a number of markings due to the attachment 

 of muscles, and therefore called muscular impressions. The two 

 largest of these are oval, and situated respectively near the front 

 and back ends. They correspond (see fig. 192) to a couple of 

 large muscles, the fibres of which run transversely across from 

 valve to valve, and which, since their contraction serves to adduct 

 or pull the valves together, are termed the anterior and posterior 

 adductor muscles (L. adduco, I lead to). When they cease to 

 contract, the elasticity of the stretched ligament comes into play 



