218 THE LAMELLIHRANCHIA 



stony consistence, whence it is called the " ossiculum " ; it projects 

 on the right side through a hole in the flattened valve of that side. 



In Modiolarca the byssogenous cavity is preceded by a second 

 glandular cavity (Fig. 241, gl.p\ but the latter takes no part in the 

 secretion of the byssus. In some cases the whole of the byssogen- 

 ous cavity degenerates in the adult, as may be seen in certain 

 species of Unio, or it is closed as in Cydas, in which genus it and 

 the byssus are highly developed during embryonic life. In the 

 endoparasite Entovalw the byssogenous apparatus appears to be 

 modified to form a so-called "organ of adhesion" (Fig. 240, f.ffl). 



The foot, with the viscera contained in it, is attached to the shell 

 by retractor muscles, of which there are normally four pairs. Two 

 pairs, the retractors and protractors, are anterior and situated near 

 the anterior adductor muscle ; one pair, the elevators, is median ; and 

 one pair of retractors is posterior and close to the posterior adductor 

 muscle. These various muscles are inserted symmetrically near the 

 dorsal border of the valves and between the two adductors. In the 

 more primitive Lamellibranchs these muscles are greatly extended in 

 a longitudinal direction (Fig. 231, f.e\ and in certain Protobranchia 

 they may form an almost continuous series ; but otherwise it is only 

 the four retractors at the extremities of the foot that are well 

 developed, the remainder being rudimentary or atrophied (Fig. 202, 

 a.f.r, p.f.r). In general, the so-called Monomyaria, or forms with a 

 single and that the posterior adductor, have only retained the 

 posterior retractors of the foot, and these muscles only exist on one 

 side in various forms that are fixed by one valve : thus in Pecten 

 only the left retractor is present, and even this is aborted in P. 

 magellanicus. When the foot becomes reduced as an organ of 

 locomotion, and, in compensation, the byssogenous apparatus assumes 

 a large size, the retractor muscles, especially the posterior pair, take 

 their origin from the latter structure, and thus become the retractor 

 muscles of the byssus. 



II. ANATOMY. 



1. The Alimentary Canal. The mouth is situated at the anterior 

 end of the body, dorsal of the base of the foot (Fig. 188, m). In 

 Solenomya (Fig. 231, m) it lies behind the anterior adductor muscle, 

 but it is on the ventral side of this muscle in all other Lamellibranchia 

 with two adductors. Except in Anomia, in which it is asymmetrical, 

 it is asymmetrical transverse aperture compressed between two lips, 

 of which one is dorsal and anterior, the other is ventral and posterior. 

 These lips have simple borders as a rule, but in the Pectinidae they 

 are scolloped and even ramified (Fig. 235, I) ; they are generally 

 continued on either side into two lobate prolongations, called the 

 labial palps, of which the external is the prolongation of the anterior 

 lip. In the genus Area (Fig. 188) the lips pass insensibly into the 



