28 BEITISH MARINE TESTACEOUS MOLLUSCA: 



The foot is a very variable organ, implanted usually in the 

 subcentral lower portion of the body ; when at rest, in many 

 families it represents a pedicle, with, an elbow doubled up at 

 its side, but in action it becomes instantly straight, pointed, 

 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 in 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 maintain 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, arising 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- 



