113 



CONCHIFERA. 



CONCHIFERA. 





climate. In its first state the oyster exhibits two semi-orbicular films 

 of transparent shell, which are continually opening and closing at 

 regular intervals. The whole brood are associated together by being 

 involved in a viscid slime, and in that state called the ' spat," it being 

 common among viviparous animals of this kind to have their spawn 

 posited in contact with the lungs. The involving slime serves as the 

 first nutriment : and we may infer that the fcetal food so influenced 

 by the gills is at the same time a respiratory supply to the imperfectly 

 formed young." In the siphoniferous branch of the family the longer 

 the siphons the larger, as a general rule, ia the mass of the ovary : in 

 those forms which have the siphons short and the foot comparatively 

 large the ovary is comparatively small. As far as anatomy has 

 hitherto detected this part of the organisation, here we have herma- 

 phroditism in the true sense of the word. The whole business of 

 reproduction is apparently carried on within the two valves of the 

 shell without the aid of a second individual, as it is in a hermaphro- 

 dite fiower. But it will occur to most observers that the Conchifara 

 are gregarious; the Fixed Conchifers (Oysters, Spondyli, Chamec, 

 Ac., for instance) eminently so ; and it is by no means clear that 

 this congregation may not be a neceisary condition for the 

 fecundation of the ova ; and that there may not be a mutual 

 diffusion of some influence analogous to that of the milt in fishes. 

 M. Prevost, who made his experiments upon the Unionist, would 

 make it appear that though there can be no union, still no propa- 

 gation takes place without an assemblage of these animals upon the 

 same spot. 



The muscular system, as it regards motion, is two-fold ; valvular 

 and locomotive. The first consists in the adaptation of muscular 

 fibre to the movement of the valves, and indeed this muscular appa- 

 ratus may in some cases be made ancillary to locomotion, as iu 

 the Pectens, for example. The adductor muscles are attached to 

 opposite points in each valve, and their office is to close the valves by 

 their contractility, or suffer them to expand by their relaxation. In 

 the greater number (itimyaria) there are two ; one anterior near the 

 oval aperture, the other posterior. The Monuinyaria have appa- 

 rently one only ; but Poli has shown that this muscle is in reality mi 

 approximation of two, and thence most probably arose the slight 

 regard manifested by Cuvier fur the division of Lamarck. The 

 second or true locomotive organ is called the foot, and is formed of 

 various layers of fibres, which by their counteraction bestow on it 

 great power of motion when the organ is well developed. Though in 

 some species merely rudimentary it is found in all the Dim 

 not so in the JUonomyaria, some of which are entirely without it. Its 

 place may be defined by stating that the mouth is generally hidden 

 between ita base and the anterior adductor. Where well developed 

 it is of various shape*, cylindrical, flattened, &c. In some it ig a 

 digging organ, or kind of ploughshare for making a furrow in the 

 sand or mud wherein the animal means to lie hid ; in others, as in 

 the cockle, Ac., it becomes a leaping organ, and enables the conchifer 

 ii> clear a boat's gunwale when laid on the bottom boards. The foot 

 is the instrument which produces the byssus. [Bvssus.] The 

 following is Deshayes'g account of the structure : " If the byssus and 

 foot of a byssiferous mollusk 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 inferior edge of the foot be inspected, a fissure will 

 be found running completely along it, at the bottom of which a 

 brownish and semicorneous 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 

 ita 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 xecretea 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 con- 

 sistency, and is firmly fixed to the stone or other body at the bottom, 

 the animal retracts ita foot, and in doing so detaches the new fibre at 

 the base of the pedicle. The mode in which the filaments of the 

 < are formed is consequently entirely different from that iu 

 which hair or the horns of the higher animals are evolved, and it is 

 easily understood when the intimate structure of the foot of the 

 byssiferous molluaks 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 nt the bottom of the 

 groove of the foot become horny, and are detached in succession in 

 the form of threads as they become consolidated." The siphons, 

 which arc the organs by which these animals take in and throw out 

 water, are retracted by means of two lateral fun-shaped muscles, 

 situated posteriorly. 



Mantle and Cuticular System. Two thin fleshy lamimc applied over 



the back of the animal, extending over its sides, and with its edges 



meeting along the anterior middle aspect of the body, covering, or 



closely in contact with, the whole interior surface of tho shell, form 



the mantle, in the thickened edge of which is the principal apparatus 



that secretes the shell : there arc also frequently rows of contractile 



ilia fringing it. The, whole, of these parts are exquisitely 



pi highly contractile. The mantle becomes free at the 



nehiic, and forms a cavity round the lower part of 



XAT. HIST. DIV. VOL. II. 



'n> 1'ietiirum. 



A, anterior or oral extremity ; P, posterior or anal extremity, with the 

 siphons or tubes ; II, Hinge ; F, Foot. 



,i . .t or II, 



lV,.lr .,,, 



=i^^-w-;-^i- "< 



A, anterior or oral extremity ; P, posterior or anal extremity. 

 A 



Shell of Ot/thcraa. 



