MORPHOLOGY OF LAMELLIBRAN CHIATE MOLLUSKS. 
425 
GENERAL CONSIDERATIONS. 
Changes in structure brought about by the degeneration of the foot. — It lias been 
demonstrated that the aortae and sinus venosus in Anodon possess valves or sphinc- 
ters, which are supposed to be operated at different times. If the foot is to be 
expanded, the sphincter of the posterior aorta closes and all the blood is driven 
through the anterior aorta into the foot. The supply comes into the heart at this 
time from the mantle. During this process, the valves of the sinus venosus are also 
closed, and the blood is confined to the foot, which it extends. 
If the animal wishes to withdraw the foot between the valves of the shell, the 
valves of the anterior aorta, which leads to the foot, are closed, and the sphincter of 
the posterior vessel extending to the mantle, as well as the valves of the sinus venosus, 
is opened. The muscles of the foot, together with the retractor muscles, now contract 
and force the blood into the sinus venosus, thence to the gills, heart, and then 
through the only open aorta, the posterior one, into the mantle. 
As has been said, Ostrea has no foot, though its ancestors must have possessed 
one, and ’the mantle, being part of the mechanism used by many forms with a large 
foot for its protrusion and retraction, has correspondingly lost most of its great blood 
spaces, which were capable of holding immense quantities of blood. The mantle has 
become firm by the substitution of compact tissue and in Mytilus and other forms by 
the reproductive glands, and its great blood supply has been reduced to the small 
amount which the animal needs for the nourishment of its tissues. The posterior 
aorta., having once supplied the mantle mainly, is now distributed instead through the 
greatly developed adductor muscle. 
I would point out the fact that there seems to be a correlation between the aborted 
or absent foot and a thick mantle with no large blood spaces; and also between a 
fully developed locomotor foot and a mantle consisting mainly of immense blood 
spaces. Pecten may be an exception to this rule, though I think it very probable that 
large blood spaces do not exist in its thin mantle. This, of course, confirms the view 
spoken of above, concerning the physiology of locomotion by the foot. It also shows 
how the condition of some of the organs in Ostrea and Mytilus has been brought 
about by the degeneration of the foot possessed by a locomotor ancestor. 
The muscle system of Mytilus. — It will be noticed from the figures that the visceral 
mass is relatively small and that a large part of it is filled by the great byssus mus- 
cles. When the fixed habit was acquired and the animal fastened itself by means of 
the byssus, these muscles were probably developed in order to prevent a tearing of 
the delicate tissues when the animal was subjected to the force of the waves. This 
injury would easily have taken place if there had been no support to the byssus by 
some attachment of it to the shell. Fig. 37 shows in section a powerful muscle on 
either side, attached to the shell dorsally. These come together on the median line, 
and their combined fibers run downward and become attached to the base of the 
byssus organ. In Fig. 42, a side view of the main muscular system, it will be seen 
that any downward pull from the byssus would bring a strain to bear on all the 
muscles whose outer ends are attached to the shell. 
Having been crowded out by the development of such a bulk of muscle tissue in 
the visceral mass, it was necessary for the generative gland to find another position, 
