308 MOLLUSCA [OH. 



lamellae of the outer gill-plate and there fertilised. They develop 

 into peculiar larvae called Glochidia, provided with a sticky thread 

 or by ss us. A bivalve shell is developed but not the foot. When 

 a fish passes by the mother expels the Glochidia from the gills, and 

 they seize hold of the tail or fins of the fish and embed themselves 

 therein. They develop there for some weeks and change gradually 

 into the adult form. They show a remarkable sensitiveness to the 

 presence of fish, but if they fail to attach themselves to one they fall 

 to the bottom of the water and perish. 



Lamellibranchiata as a group have very uniform habits : the 

 principal points in which they differ from one another are (1) the 

 degree of complexity which that all-important organ the ctenidium 

 has attained, and (2) the extent to which the animal is able to burrow. 



The simplest forms, such as Nucula, have ctenidia like those of 

 a Gastropod, a fact which suggests the view now generally held, 

 that the Lamellibranchs are descended from some primitive type of 

 Gastropod. 



In others, such as the Sea-mussel (Mytilus), the ctenidia have 

 the same external appearance as those of Anodonta, but the filaments 

 are very loosely united with one another; they adhere to one another 

 in fact merely by the interlocking of lateral bunches of cilia and 

 their upturned ends are not fused to the mantle. The foot is small 

 and tongue-shaped, the animal never burrows and seldom moves, 

 being fixed by a cord of mucus called the byssus, secreted by a 

 gland in the hinder part of the foot. 



In the Oyster (Ostrea) the foot has disappeared and the animal 

 passes its life resting on one side. In the Scallop (Pecten) the foot 

 has also atrophied, but the animal is able to swim through the 

 water by flapping the valves of the shell. The Cockle (Cardium) 

 has a large and powerful foot by which it is enabled to execute leaps. 



Mya (sometimes known as the Clam, though this term is applied 

 to many Bivalves) and its allies burrow deeply in the sand and 

 have the edges of the mantle behind drawn out into two long tubes 

 closely apposed to one another, termed the dorsal and ventral 

 siphons. By means of these tubes they keep up a connection 

 with the surface, so that the currents of water are not interrupted. 

 Similar tubular funnels, though not so much drawn out, are seen in 

 the Razor-shell (Solen) (Fig. 141). Pholas and some others are able 

 to burrow in rock ; this is said in some cases to be effected by an 

 acid secretion poured out by the flat disc-like end of the cylindrical 

 foot. Teredo, the ship-worm, burrows in timber; the siphons are very 

 long and covered with a shelly deposit ; the original valves of the 



