84 Invertebrata. 



Many bivalves are fixed in the adult state ; in the 

 oyster, scallop, &c., the animal lies on one side, the 

 under shell adhering to the surface on which it rests. 

 The common mussel is fixed by means of an anchor- 

 age of strong fibres (called byssus) secreted by a 

 gland on its foot. Under each lobe of the mantle 

 lie the lamellar gills, between which is a fleshy pro- 

 trusion, the foot or organ of locomotion. At the 

 front is the mouth (fig. 49, f) from which the diges- 

 tive tube is continued backward, to open above the 

 posterior adductor muscle as seen in the sketch. 

 The last portion of this digestive tube passes right 

 through the cavity of the heart. The siphon, when 

 it exists, is a double tube, consisting of an upper 

 and a lower passage ; through the latter the food and 

 water for breathing purposes enter into the mantle 

 cavity and bathe the gills ; through the upper tube 

 the excreted matter and the water returning from the 

 gills are ejected. There is a nerve ganglion above 

 and below the digestive canal at the base of the little 

 lobes around the mouth (* labial tentacles,' seen in 

 fig. 49 as lancet-like processes), and another exists 

 in the foot below the digestive tube ; a fourth is 

 placed posteriorly beneath the hinder adductor muscle. 

 These animals have no recognisable head, but some 

 of them have eyes on the siphon, as in the razor- 

 shell (Solen); others have eyes along the edge of 

 the. mantle lobes, as in the common scallop. The 

 larvae of all bivalves have eyes, but these are lost 

 in the course of development, and when such organs 

 appear in the adult they are, of secondary forma- 

 tion. 



