218 THE LAMELLIBRANCHIA 
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.y), 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 Cyclas, in which genus it and 
the byssus are highly developed during embryonic life. In the 
endoparasite Lntovalva the byssogenous apparatus appears to be 
modified to form a so-called “organ of adhesion” (Fig. 240, f.g/). 
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, fe), 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, 
afr’, pr). In general, the so-called Monomyaria, or forms with a 
single and that the posterior adducter, 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. 
IJ. ANATOMY. 
1. The Alimentary Canal.—The month 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, 1); 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 4rca (Fig. 188) the lips pass insensibly into the 
