28 BRITISH MARINE TESTACEOUS MOLLUSCA: 
The foot is a very variable organ, implanted usually in the 
subcentral lower portion of the body; when at rest, im many 
families it represents a pedicle, with an elbow doubled up at 
its side, but in action it becomes instantly straight, poimted, 
flexible, and extensible more or less in all directions, full ‘of 
energy and activity. Many of these animals have at the bend 
at the pedicle a byssal groove, from which tenacious filaments 
are spun, that enable the animal to fix itself in situations 
where such action is required, and detach again, which is 
always effected by the extraction of the byssus from its origin, 
leaving it fixed at the last locality, and when necessary, spin- 
ning another with great celerity; not as a matter of course, 
but im case circumstances, as currents and rapid tides, require 
such aid. 
As to the nervous masses, they are few and scanty, a single 
ganglion being placed above the mouth, the other posteriorly ; 
but they are connected with each other by filaments, from 
which threads ramify to all the muscular supports of the body, 
particularly those important ones the adductors, the foot, 
siphonal retractors and minor muscles: the muscular compo- 
sition of these organs appears in their textures under every 
imaginable superficial, horizontal, and perpendicular angle. 
As the shell or hard parts of the bivalves are essential ele- 
ments of their composition, I am bound to give them some 
consideration ; I shall however only make a few remarks on 
the ligament and cartilage, as well as the teeth; the former 
is chiefly intended, aided by the teeth, to maimtain the valves 
in a symmetrical position, when the cartilage is unbent, by 
the relaxation of the adductor muscles. The cartilage is the 
great source of elasticity, arismg from the impacted mass of 
fibres of which it is composed, that give its substance almost 
the appearance of homogeneity. When the animal closes 
the shell by the retractive action of the adductors, its spring 
is then bent, and on relaxation the valves are opened to the 
extent the adductors are relaxed; but if they are divided by 
the knife, the elastic power of the cartilage opens them to the 
fullest extent, at least until they are stopped by the beaks and 
umbones. Whether the cartilage is termed external or in- 
