MOLLUSCA PEOPER. 



177 



lastly, the shell is more developed on one side than the other, 

 so as to become "inequilateral" (Fig. 81, 2). The lobes of 

 the mantle are sometimes 

 quite free ; but, at other times, 

 they are more or less united 

 to each other, and leave only 

 two openings. Through one 

 of these openings (the ante- 

 rior) the " foot " is protruded 

 (Fig. 80, /); and through 

 the other pass the respiratory 

 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 

 flattened disk, as in the ordi- 

 nary univalves. In many 

 cases, it is quite rudimentary ; 

 and even when it is employed 

 in locomotion it is usually 

 small. Most generally, it is 

 hatchet-shaped or pointed 

 (Fig. 80, /), and serves 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, which 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 

 object. Besides the muscular 

 foot, other muscles are pres- 

 ent as well in the Lamelli- 

 branchiata. Of these, the 

 most important are the 

 muscles which close the shell, 

 and are called the " adduc- 

 tor " muscles. In one group of the bivalves (Fig. 81, 3), there 



valve and mantle-lobe, and half the siphons 

 are removed. s Respiratory siphons, th4 

 arrows indicating the direction of the cur- 

 rents ; a a' Adductor muscles ; 6 Gills ; h 

 Heart ; o Mouth, surrounded by ( p) labial 

 palpi ; / Foot : v Anus ; m Cut edge of the 

 mantle. 



