MOLLUSCA PEOPEK. 



19'; 



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

 is protruded (fig. 137, /) ; and 

 througli the other pass tlie res- 

 piratory tubes or "siphons" (4). 

 The foot in tlie Bivalves is a mus- 

 cular organ developed upon the 

 lower surface of the bod}', 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 liatchet-shapeil or 

 jjointed (fig. 137, /), and sei'ves 

 to enable the animal to make 

 short leaps. In many cases — as 

 in the common Mussels — the foot 

 is subsidiary to a special gland, 

 which secretes a viscous fluid, 

 that hardens rapidly on exjjosure 

 to the air. This fluid is moiilded 

 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 ai-e pjresent as well 

 in the Lamellihranehiata. 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 pireseut 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 

 14 



Fig. 137— Anatomy of a Bh'alve Mollusc. 

 iljia arenaria (afiev Woodward). The 

 left valve and mantle-lobe, and lialf the 

 siphons, are removed, s 5 Respinitory 

 siphons, the arrows indicating the direc- 

 tion of tiie currents ; a ct' Adductor 

 muscles ; b Gills ; 1 Heart ; o Mouth, 

 surrounded by (p) laliial palpi ; /Foot ; V 

 Anus ; 711 Cut edge of the mantle. 



