MOLLUSCA. LAMELLIBRANCHTA 199 



animal to jump to a considerable distance. In the genus 

 Mytilus the foot is very much reduced ; in others which 

 have lost the power of locomotion {e.g. Ostrea) it is absent 

 altogether. On the posterior part of the foot there is in 

 some genera {e.g. Mytilus, Pinna, Area) 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. 91 g); these consist of filaments which usually be- 

 come connected so as to form leaf- or plate-like bodies, 

 whence the name Lamellibranchia. 



In some forms the margins of the two mantle-lobes 

 although in contact are not united, and when this is the 

 case there are usually at the posterior margin two open- 

 ings leading from the exterior to the mantle-cavity; these 

 are produced by adjoining excavations or notches in the 

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

 however, the two lobes of the mantle are fused at one or 

 more points ; this union occurs between the exhalent 

 and inhalent openings, and also, in many forms, below 

 the latter opening. In this way the mantle becomes a 

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

 protrusion of the foot, and two posterior for the inhalent 

 and exhalent currents of water. Frequently, at the 

 posterior openings, the mantle is greatly produced so as 

 to form two complete tubes, known as siphons (fig. 91, s, 

 s') ; these are sometimes free, sometimes united, and may 

 be as much as four times the length of the shell. The 



