314 



Mollusca. 



feed upon small organisms^ Diatoms, etc., which are contained in the 

 inhaled water. They can crawl slowly along by means of the foot, 

 which they press upon or into the ground ; some can take leaps by 

 its means. In exceptional cases they can move rapidly through 

 the water by quickly opening and shutting the shell. Some 

 are able to work themselves into the soft sandy, or muddy ground 

 by means of the foot, so that the cloacal and inhalent apertures alone 

 project, and many pass most of their lives buried in this way ; the 

 formation of shorter or longer syphons is in correlation with this. 

 Some even have the power of boring into hard substances, wood, 

 limestone, etc., grinding the material away with the foot in which 

 tiny silicious bodies (?) are embedded. Some forms have the shell 

 firmly attached to the surroundings by means of a calcareous 

 secretion, and have, of course, lost the power of locomotion. As 

 has been mentioned above, such an attachment may be also 

 effected by the byssus thread ; the animals so attached remain for a 

 long time in the same place, but may leave it by throwing off the 

 byssus ; the Dreissena mentioned below is, for example, attached 

 during summer to foreign bodies just below the surface of the water, 

 in the autumn the byssus is discarded and the animal retreats to 

 the bottom. 



As examples the following may be mentioned : 



1. The Oystei- (Osirea edwZis) possesses only one adductor ; the ligament 

 is internal ; the foot absent ; the right valve flat, the left convex and adherent to 

 various foreign bodies : on English coasts. Related to the Oysters are the 

 Scallops (Pecten) with a radially ribbed shell, of which the anterior and 

 posterior halves are similar. In several species the left valve is flat, the right 

 convex ; in others both are alike convex ; internal ligament ; eyes at the edge 

 of the mantle ; one adductor ; small foot : species in the North Sea, English 

 Channel, etc. 



2. The Mussel (Mytilus edulis), characterised by the position of the 

 umbo at the anterior end of the rather thin shell ; and possessing a long internal 

 ligament and a powerful byssus, by means of which the animal attaches itself to 

 stones, etc. . very abundant on British coasts ; if it lives in stagnant water 

 (harbours) a poisonous substance is secreted by the liver. The allied, some- 

 what smaller Dreissensia polymorpha, inhabits fresh water ; originally a native of 

 South-East Europe, it has spread during the present centuiy over the whole 

 continent. Another allied form is the almost cylindi-ical, elongate Lithodomus 

 lithophagus, which bores into limestone ; a MediteiTanean form. 



Fig. 260. Mya arenaria. f foot, 

 Meyer and Mobius. 



cloacal opening, u respiratory opening. — After 



