MOLLUSCA 



245 



surface of the mantle, are richly ciliated. There is no distinct 

 head ; on each side of the mouth are two delicate folds, the 



FlG. 253. Scrobicularia piperata, in its natural position, partly buried in the sand. A 

 exhalant siphon ; B, inhalant siphon. (From the- Cambridge Natural History.) 



labial palps, which assist in carrying food to the mouth. On 

 the ventral side of the body at the anterior end is the foot, 

 a muscular structure, often laterally flattened with a pointed 

 extremity; it varies very greatly in size and shape in differ- 

 ent species ; in some it is very much reduced or even entirely 



l.m, rcb y ait 



a.ccd. 



I- ejct.plp 



l.Lnt.fjljb c - 



J "v.U U ^^ 



,*>-<*oL 



r.Trz, 



FlG. 254. Anodonta cygnea. The left valve of the shell, and the greater part of the left 

 mantle lobe, removed, a, anus ; a. ad, anterior adductor; a. r, anterior retractor; an, left 

 auricle; d.p.a, dorsal pallial aperture; ex.sph, exhalant siphon; ft, foot; in.sph, inhalant 

 siphon; id, kidney; l.ext.gl,\eft external gill; Lext.plp, left external labial palp; l.int.gl, 

 left internal gill; I. int. pip, left internal labial palp ; l.m, cut edge of the left mantle lube; 

 mtk, mouth; p.i id, posterior adductor; pc, pericardium ; pr, posterior retractor; pre, pro- 

 tractor; ret, rectum; r.m, right mantle lobe; v, ventricle; v.m, visceral mass. (After 

 Parker and Haswell.) 



wanting 



genera is a 



Connected with the foot in some 

 gland, the byssus gland, which secretes a substance forming 

 threads, usually brownish in color, by means of which the 

 animal attaches itself to other objects (Fig. 255). In some 

 cases at least this attachment is not permanent, for the animal 



