124 ORGANS OF ATTACHMENT, 



tends by its elasticity to separate the two valves between 

 which it is placed. The eight transverse plates composing 

 the shell of the chitons have generally a strong coriaceous 

 ligamentous band connecting their margins, and secreted 

 like the ligaments of conchifera, by the surface of the skin. 

 The byssus of conchifera consists of horny filaments se- 

 creted in a fluid state by a gland behind the base of the foot; 

 they are conveyed along a median groove of the foot, to be 

 attached to external solid bodies, to anchor the more delicate 

 shells, as pinna and other mytilacious bivalves. The horny 

 operculum of most of the testaceous turbinated gasteropods 

 is of a condensed albuminous nature, secreted in successive 

 superimposed layers added to the attached surface by the 

 muscular foot, to which it adheres in the same manner as 

 the retractor muscle adheres to the columella of the shell. 

 From the ginglimoid hinge of the shells of conchiferous 

 mollusca admitting only of flexion and extension in one 

 direction, few, though powerful, muscles are required to 

 effect their motions ; but the joints of the articulated classes 

 of animals being generally formed by a segment of one 

 sphere being enclosed within another, and consequently ad- 

 mitting of rotation in every direction, numerous muscles are 

 required to effect their varied movements. The ginglimoid 

 crural joints of the entomoid classes being formed by hollow, 

 cutaneous, solid tubes, have generally one very strong and 

 broad flexor tendon, and a more narrow and feeble extensor 

 tendon, into which all the muscles of the joint are inserted, 

 and they are deeply excavated and covered only by the 

 unconsolidated, thin, tough epidermis and skin on the side 

 to which the joints are inflected. There are no opercula, or 

 moveable pieces of the shells in the pteropoda or cephalo- 

 poda, and the soft cartilaginous, organized, internal bones of 

 the cephalopods are connected only by muscles, and by thin 

 unconsolidated portions of their own soft matter. 



In the soft, flexible, cartilaginous skeletons of the lowest 

 cyclostome fishes, there are scarcely any traces of articula- 

 tions or ligaments, and in the higher chondropterygii they 

 are almost confined to the most moveable parts connected 

 with mastication, and the fins for progressive motion. In 

 fishes, as in the inferior classes, where the skeleton becomes 

 more consolidated by earthy depositions, the articulations 



