MOLLUSCA PEOPER. 



197 



openings. Through one of these openings (the anterior) the " foot " 

 is protruded (fig. 137, /) ; and 

 through the other pass the res- 

 piratory tubes or "siphons" (s). 

 The foot in the Bivalves is a mus- 

 cular organ developed upon the 

 lower surface of the body, but 

 not forming a creeping flat- 

 tened disc as in the ordinary 

 Univalves. In many cases it 

 is quite rudimentary ; and even 

 when it is employed in locomo- 

 tion it is usually small. Most 

 generally it is hatchet-shaped or 

 pointed (fig. 137, /), and serves 

 to enable the animal to make 

 short leaps. In many cases — as 

 in the common Mussels — the foot 

 is svibsidiary to a special gland, 

 which secretes a viscous fluid, 

 that hardens rapidly on exposure 

 to the air. This fluid is moulded 

 by the foot into silky threads 

 (the so - called " byssus "), by 

 means of which the shell is 

 firmly fixed to some solid ob- 

 ject. Besides the muscular foot, 

 other muscles are present as well 

 in the LamellihrancMata. Of 

 these the most important are the 

 muscles which close the shell, 

 and are called the " adductor " 

 muscles. In one group of the 

 Bivalves {e.g., Oyster, Scallop, 

 &c.), there is only one adductor 

 muscle, but ordinarily there are 

 two (fig. 138). These muscles 

 leave distinct scars or " muscular 

 impressions" in the dead shell, 

 so that it is easy to determine 

 how many were present in any 

 given shell. The margin of the 

 mantle, too, is muscular, and 

 leaves upon the shell a distinct line where it was attached, this 



Fig. 137. — Anatomy of a Bivalve Mollusc. 

 Mya areimria (after Woodward). Tlie 

 left valve and mantle-lobe, and half the 

 siplions, are removed, s s Respiratory 

 siphons, the arrows indicating the direc- 

 tion of the currents ; a a' Adductor 

 muscles ; & Gills ; h Heart ; o Mouth, 

 surrounded by (p) labial palpi ; /Foot ; v 

 Anus ; m Cut edge of the mantle. 



