MORPHOLOGY OF LAMELLIBRANCHIA.TE MOLLUSKS. 
393 
The muscle system of Mytilus edulis is worthy of remark. Fig. 42, PI. lxxxv, repre- 
sents the musculature dissected out entire. From the base of the byssus at its ante- 
rior portion, two cylindrical muscles, called the anterior foot-retractors, run forward, 
passing on either side of the mouth (Fig. 33, ar), and are attached to the valves of the 
shell. They are, in the adult, entirely free from any muscles connected with the foot 
and all their fibers are inserted in the base of the byssus. The tongue-like foot, with 
its concave anterior surface (Fig. 42, /), is continued directly upward to the shell, as 
two cylindrical muscle bundles (Fig. 42, /to), which pass exteriorly to the two anterior 
muscles just mentioned (Fig. 36, /to and ar). The foot with these muscles may be 
removed without disturbing any of the other muscles. These are the posterior foot- 
retractors. The byssus organ is morphologically a part of the foot, and these muscles 
described are probably the anterior and posterior foot-retractors, respectively. But 
the byssus organ has lost all connection with the musculature of the foot, as have the 
anterior adductors also. 
From the base of the byssus, just behind the insertion of the two anterior muscles, 
extends a great mass of muscle bundles which, attaching themselves to the shell above 
and posteriorly, serve as its main support. These byssus muscles are arranged in two 
groups. They are close to one another at the byssus and diverge laterally above to 
become attached to the valve of either side. (Shown in section in Fig. 37 at bm.) The 
mass extends obliquely backward and is divided with much regularity into a number 
of large bundles (Fig. 42 bm). They are shown in the vertical sections at bm in Figs. 
38 to 40. 
A dorsal muscle, well shown in JYucula and Solenomya, occupies a position nearly 
parallel to the anterior foot-retractors (Pelseneer, No. 17, Figs. 7 and 15), and its attach- 
ment to the shell is slightly posterior to the latter. In Pecten the foot-retractors, and 
in Anomia the byssus muscles, are attached only to the left valve. 
Other large muscles are developed in the mantle of many lamellibranchs, in the 
region of the siphon, which will be spoken of in that connection. 
THE FOOT. 
This characteristic organ, appearing generally, though sometimes greatly modi- 
fied, throughout the Mollusca, is in lamellibranchs a muscular projection from the 
ventral surface of the main body of the animal. It extends more or less anteriorly. 
In the primitive forms Yoldia and Solenomya , the lower part of the foot is tur ned nearly 
forward, and where the organ has degenerated greatly, as in Pecten ir radians, it arises 
from the extreme anterior end of the visceral mass. In Mya arenaria it is relatively 
small, though functional in locomotion and situated far forward. In Venus mercenaria 
and many others, it occupies the whole ventral surface of the visceral mass, extending 
slightly backward as well as forward, and in a few forms there is a greatly developed 
heel-like projection posteriorly. 
Certain simple and probably primitive lamellibranchs possess a foot which has a 
more or less circular ventral disc ( Pectunculus , JYueulidce , Solenomyidce). Around the 
margin of this are a number of thick, short papillse or flutings. Fig. 52, PL lxxxvii, 
representing Yoldia limatula with the right valve and mantle fold removed, will show 
the general relations of the foot (/). It is seen to have a volume greater than all the 
rest of the animal, the mantle being very thin. The ventral disc (d) is not expanded 
