702 



C6NCHIFERA. 



parietes of the foot are preserved in immediate 

 communication by means of a great number of 

 small muscles, sometimes straight, sometimes 

 oblique, and variously interlaced, to which 

 Poli has given the name of funicular muscles 

 (j>j> fig- 347). They are particularly conspi- 

 cuous in the cylindrical foot of the Solens, in 

 the flattened foot of the Tellinse, and of the 

 Uniones, and they have a remarkable arrange- 

 ment in that of the Cardiae. They appear to 

 be wanting in the foot of those Conchiferous 

 mollusks that attach themselves by means of a 

 byssus. In them the foot is reduced to the 

 functions of spinning (de filer) the threads of 

 the byssus, and it is not therefore surprising 

 that its organization should be found to be 

 peculiar. Reduced to a purely rudimentary 

 state, the foot in the Monomyaria (b, Jig. 348) 

 appears rather as an appendage to the mass 

 of the viscera than as their defensive envelope. 

 The muscular fasciculi that terminate it pos- 

 teriorly are small; they pass through the vis- 

 ceral mass to be attached either to the superior 

 part of the central muscle, or within the in- 

 terior of the hooks or beaks of the shell. 

 Almost the whole of the Monomyaria furnished 

 with a foot, have a byssus also ; to this rule 

 there are indeed a small number of exceptions, 

 among others the Limae. 



Up to the present time the faculty of pro- 

 ducing a byssus is not known to belong to 

 any other class of animals, and it is limited 

 to a few only of the Conchiferous mollusks. 

 Among the Dimyaria the genus Byssomya may 

 be quoted as an example, also the members of 

 the family of the Mytilacea ; and, if the horny 

 plates of certain Archse be likened to the 



Fig. 353. 



byssus, it would also be necessary to include 

 this genus in the group of byssiferous Dimyaria. 

 In the Monomyaria provided with a foot, the 

 whole of the genera are byssiferous, with the 

 exception of those which attach themselves im- 

 mediately by their shell. 



The byssus (b,jig. 353) is a bundle of horny 

 or silky filaments, of different degrees of fine- 

 ness and of different thicknesses, and flexible 

 in various measures, by means of which the 

 animal is, as it were, anchored to any solid 

 body sunk in the sea. The filaments, for the 

 most part distinct from one another, are, how- 

 ever, occasionally connected into a single mass 

 of a subcylindrical form, and terminated by a 

 broad expansion, which serves as the point 

 of attachment. This disposition is to be ob- 

 served in the Aviculse, and leads to the belief 

 that the horny mass of certain Archae is a mere 

 modification of the byssus. In those species of 

 which a byssus is formed of separate filaments, 

 these are all seen to be detached from a com- 

 mon pedicle (c,,fig. 353), situated at the infe- 

 rior base of the foot (d, Jig. 353). If the 

 byssus be examined before any of the filaments 

 are torn, it is easy to perceive that these are 

 attached to submarine bodies by means of a 

 small disc-like expansion of their extremities, 

 of various extent according to the genus and 

 species (a, a, a, Jig. 354). Attentive examina- 

 tion of these filaments shews that they are of 

 equal thickness through their entire length, and 

 that they have nothing of the structure of the 

 hair of the higher animals. 



If the byssus and foot of a byssiferous mol- 

 lusk be placed under a powerful lens, the last 

 filaments of the byssus are first seen to be 

 nearest to the base of the foot; and if the infe- 

 rior edge^ of the foot be inspected, a fissure will 



