CONCHIFERA. 



703 



be found running completely along it, at the 

 bottom of which a brownish and semi-corneous 

 filament is often to be perceived ; this is neither 

 more nor less than a filament of the byssus 

 prepared to be detached by the animal, in 

 order to which the animal stretches forth its 

 foot until it encounters the object upon which 

 the other fibres of the byssus are fixed ; to this 

 it applies the point of the foot which then se- 

 cretes a small quantity of glutinous matter, 

 continuous with the silky filament lying along 

 the bottom of the furrow of which we have 

 spoken. When the pasty matter has acquired 

 sufficient consistency, and is firmly fixed to the 

 stone or other body at the bottom, the animal 

 retracts its foot, and in doing so detaches the 

 new fibre to the base of the pedicle. The 

 mode in which the filaments of the byssus are 

 formed, is consequently entirely different from 

 that in which hair or the horns of the higher 

 animals are evolved, and it is easily under- 

 stood when the intimate structure of the foot of 

 the byssiferous mollusks is known, when we 

 are aware that this organ consists in its centre 

 of a pretty considerable fasciculus of parallel 

 and longitudinal fibres. By a faculty peculiar 

 to the class of animals that now engages our 

 attention, the fibres situated at the bottom of 

 the groove of the foot become horny, and are 

 detached in succession in the form of threads 

 as they become consolidated. Certain genera 

 are celebrated for the abundance and fineness 

 of the byssus ; that of the Pinnae, among others, 

 which was even known to the ancients, may be 

 spun into threads like silk or wool, and may be 

 used to manufacture tissues of an unchangeable 

 colour, and of great strength and durability. 



With reference to form, the foot presents a 

 variety of interesting modifications. Some- 

 times it is short and truncated, as in the genus 

 Pholas; sometimes more elongated, but still 

 truncated at the summit, 

 Fig. 356. as | n certain Razor-shells 

 (Solen), (a, fig. 355) ; 

 in which the edges of 

 the truncation are regu- 

 larly toothed. A few of 

 the acephalous mollusks 

 have the foot cylindrical 

 (, Jig. 356), as the So- 

 lencs ; when it presents 

 this form, the organ is 

 generally terminated by 

 a kind of glutinous point, 

 or disc, which enables 

 the animal to fix itself at 

 different heights in the 

 deep cylindrical hole it 

 digs for itself in the 

 sand. The foot, which 

 is shaped like a tongue, 

 is named linguiform, as 

 in the Solen strigilatus; 

 it is cluviform when it 

 is thicker at its extremity 

 than at its base : it is 

 found of this shape in 

 certain other Solens. The 



Fig. 355. 



foot again is vermiform when it is very slender 

 and much elongated, as in the Loripes and 

 Lima. When it is thus formed, it appears 

 to us to be incapable of subserving motion. 

 In a considerable number of species the foot 

 is conical, as in the Cockle, (a, Jig- 357) ; 

 and in this case it is generally folded into 

 two nearly equal portions, so that by its means 



Fig. 357. 



the animal can leap pretty actively. It is secu- 

 riform when its free edge is arched like the 

 cutting face of an axe, as in Petuncttlus, (a, 

 Jig. 358). When it presents this form its edge 



Fig. 358. 



is generally divided into two lips, which, being 

 separated, present with some degree of ac- 

 curacy, although much contracted, the sem- 

 blance of the locomotive plane of certain Gas- 

 teropoda. When this structure occurs, the 



Fig. 359. 



foot is said to be bifid, as in Nucitlu, Trigonia. 

 It is said to be flattened when it is thin and 

 laterally depressed, as in Tellina and Donax ; 

 to conclude, it is designated as bent when it 

 consists of two portions connected at an angle 

 with one another (b, fig. 359), of which the 

 genera Cardium, Nncula, and Trigonia present 

 examples. Various other modifications, of less 

 importance than those we have particularized, 



