128 MOLLUSCA. LAMELLIBRANCHIATA. 



enables the animal to jump to a considerable distance. 

 In the genus Mytilus the foot is very much reduced ; in 

 others (e.g. Ostrea) it is absent altogether, but this occurs 

 only in those forms which have lost the power of loco- 

 motion. In some genera (e.g. Mytilus) there is on the 

 posterior part of the foot a gland which secretes a bundle 

 of horny fibres, known as the byssus, by means of which 

 the animal moors itself to foreign objects. On each side 

 of the foot, between it and the mantle, and attached to 

 the body dorsally, are the gills or branchiae (fig. 44 g). 

 These are usually leaf-like, hence the name Lamelli- 

 branchiata; there are two pairs, one on each side of the 

 body, and they are covered with cilia. 



In some forms, like Anodon, the margins of the two 

 mantle-lobes although in contact are not united, and 

 when this is the case, there are at the posterior margin, 

 two adjoining openings leading from the exterior to the 

 mantle- cavity ; these are produced by the apposition of 

 contiguous excavations in the mantle-lobes. A current 

 of water caused by the cilia on the gills and mantle flows 

 in through the ventral opening, and provides the animal 

 with food and oxygen ; another current flows out through 

 the dorsal opening, carrying with it faecal and indigestible 

 matters. In many cases, however, the two lobes of the 

 mantle are fused to a greater or less extent at their 

 margins. This union occurs at two places, one between 

 the exhalent and inhalent openings, and the other ventral 

 to the latter opening. In this way the mantle comes to 

 form a kind of bag, having three openings, a ventral for 

 the protrusion of the foot, and two posterior for the in- 

 halent and exhalent currents of water. Frequently the 

 mantle is greatly produced at the posterior openings, so 

 as to form two complete tubes, known as siphons (fig. 44 



