150 



MOLLUSCA 



that the Monomya have been developed from Isomya- 

 like ancestors, and have lost by atrophy their anterior 

 adductor. The single adductor muscle of the Monomya 

 is separated by a 

 difference of fibre 

 into two portions, 

 but neither of these 

 can be regarded as 

 possibly represent 

 ing the anterior 

 adductor of the 

 other Lamelli- 

 branchs. One of 

 these portions is 

 more ligamentous, 

 and serves to keep 

 the two shells con- 

 stantly attached to 



one another, whilst Flo. 127. Left valve of the same shell from the inner 



the more fleshypor- 



tion serves to close the shell rapidly when it has been gaping. 

 In removing the valves of the shell from an Anodon, it 

 is necessary not only to cut through the muscular attach- 

 ments of the body-wall to the shell but to sever also a 

 strong elastic ligament, or spring resembling india-rubber, 

 joining the two shells about the umbonal area. The shell 

 of Anodon does not present these parts in the most strongly 

 marked condition, and accordingly our figures (figs. 125, 

 126, 127) represent the valves of the Sinupalliate genus 

 Cytherea. The corresponding parts are recognizable in 

 Anodon. Eeferring to the figures (125, 126) for an ex- 

 planation of terms applicable to the parts of the valve and 

 the markings on its inner surface corresponding to the 

 muscular area which we have already noted on the surface 

 of the animal's body we must specially note here the posi- 

 tion of that denticulated thickening of the dorsal margin 

 of the valve which is called the hinge (fig. 127). By this 

 hinge one valve is closely fitted to the other. Below this 

 hinge each shell becomes concave, above it each shell rises a 

 little to form the umbo, and it is into this ridge-like upgrowth 

 of each valve that the elastic ligament or spring is fixed (fig. 

 127). As shown in the diagram (fig. 

 127 :f ) representing a transverse sec- 

 tion of the two valves of a Lamelli- 

 branch, the two shells form a double 

 lever, of which the toothed-hinged is 

 the fulcrum. The adductor muscles 

 placed in the concavity of the shells a 

 act upon the long arms of the lever 

 at a mechanical advantage ; their con- 

 traction keeps the shells shut, and 

 stretches the ligament or spring h. 

 On the other hand, the ligament /( 

 acts upon the short arm formed by 

 the umbonal ridge of the shells ; when- 

 ever the adductors relax, the elastic 

 substance of the ligament contracts, 

 and the shells gape. It is on this 

 account that the valves of a dead La- 

 mellibranch always gape ; the elastic 

 ligament is no longer counteracted by 

 the effort of the adductors. The state 

 of closure of the valves of the shell is 

 not, therefore, one of rest ; when it is 

 at rest that is, when there is no 

 muscular effort the valves of a Lamellibranch are slightly 

 gaping, and are closed by the action of the adductors when 

 the animal is disturbed. The ligament is simple in Anodon ; 

 in many Lamellibranchs it is separated into two layers, an 

 outer and an inner (thicker and denser). That the condition 



shells, ligament, and ad- 

 ductor muscle, a, 6, right 

 and left valves of the 

 shell ; c, (I, the ombonos 

 or short ai ms of the lever ; 

 e, f, the long arms of the 

 lever; fj, the hin^r ; h, Ihc 

 ]i-:uiir!it ; i, the adductor 

 muscle. 



of gaping of the shell-valves is essential to the life of the 

 Lamellibranch appears from the fact that food to nourish 

 it, water to aerate its blood, and spermatozoa to fertilize 

 its eggs, are all introduced into this gaping chamber by 

 currents of water, which are set going by the highly- 

 developed ctenidia. The current of water enters into the 

 sub-pallial space at the spot marked e in fig. 124, (1), 

 and, after passing as far forward as the mouth w in fig. 1 24, 

 (5), takes an outward course and leaves the sub-pallial 

 space by the upper notch d. These notches are known 

 in Anodon as the afferent and efferent siphonal notches 

 respectively, and correspond to the long tube-like afferent 

 inferior and efferent superior " siphons " formed by the 

 mantle in many other Lamellibranchs (fig. 130). 



Whilst the valves of the shell are equal in Anodon we 

 find in many Lamellibranchs (Ostrasa, Chama, Corbula, etc.) 

 one valve larger, and the other smaller and sometimes flat, 

 whilst the larger shell may be fixed to rock or to stones 

 (Ostrsea, &c.). A further variation consists in the develop- 

 ment of additional shelly plates upon the dorsal line be- 

 tween the two large valves (Pholadidaj). In P/iolas dadylus 

 we find a pair of umbonal plates, a dors-umbonal plate and 

 a dorsal plate. It is to be remembered that the whole of the 

 cuticular hard product produced on the dorsal surface and 

 on the mantle-flaps is to be regarded as the "shell," of 

 which a median band-like area, the ligament, usually remains 

 uncalcified, so as to result in the production of two valves 

 united by the elastic ligament. But the shelly substance 

 does not always in boring forms adhere to this form after 

 its first growth. In Aspergillum the whole of the tubular 

 mantle area secretes a continuous shelly 

 tube, although in the young condition two 

 valves were present. These are seen (fig. 

 129) set in the firm substance of the adult 

 tubular shell, which has even replaced the 

 ligament, so that the tube is complete. In 

 Teredo a similar tube is formed as the animal 

 elongates (boring in wood), the original shell- 

 valves not adhering to it but remaining mov- 

 able and provided with a special muscular 

 apparatus in place of a ligament. 



Let us now examine the organs which lie 

 beneath the mantle-skirt of Anodon, and are 

 bathed by the current of water which cir- 



Fig. 12S. Fig. 129. 



Fin. 128. Shell of Aspergillum mginijeritm(trum Owen). 



Fio. 12'.'. Shell of Asi:<'ryilhim v<njintfcrinn to show the original valves n, now 

 embedded in a continuous calcification of tubular form (from Owen). 



culates through it. This can be done by lifting up and 

 throwing back the left half of the mantle-skirt as is re- 

 presented in fig. 124, (3). We thus expose the plough- 

 like foot ( /'), the two left labial tentacles, and the two 

 left gill-plates or left ctenidium. In fig. 124, (5), one of 

 the labial tentacles n is also thrown back so as to show 



