MOLLUSCA 



149 



other Lamellibraucliia. The Swan Mussel has superficially 

 a perfectly-developed bilateral symmetry. The left side of 

 the animal is seen as when removed from its shell in fig. 

 124 (1). The valves of the shell have been removed by 

 severing their adhesions to the muscular arese h, i, k, I, m, u. 



Fig. 124. — Diagrams of the external form and anatomy of Aiwdonta cygTiea, the 

 Pond- Mussel ; in all the figures the animal is seen from the left side, the 

 centro-dorsal region uppermost, as in the drawings of fig. 75, which compare. 

 (1) Animal removed from its shell, a prohe g passed into the sub-pallial 

 chamber through the excurrent siphonal notch. (2) View from the ventral 

 surface of an Anodon with its foot expanded and issuing from between the 

 gaping shells. (3) The left mantle-flap reflected upwards so as to expose the 

 sides of the body. (4) Diagrammatic section of Anodon to show the course of 

 the alimentary canal. (5) The two glU-plates of the left side reflected upwards 

 so as to expose the fissure between foot and gill where the probe g passes. 

 (6) Diagram to show the positions of the nerve-ganglia, heart, and nephridia. 

 Letters in all the figures as follows : — a, centro-dorsal area ; 6, margin of 

 the left mantle-flap ; c, margin of the right mantle-flap ; d, excurrent siphonal 

 notch of the mantle margin ; e, incuri'ent siphonal notch of the mantle 

 margin ; f, foot ; g, probe passed into the superior division of the sub-pallial 

 chamber through the excurrent siphonal notch, and issuing by the side of 

 the foot into the inferior division of the sub-pallial chamber ; \ anterior 

 (pallial) adductor muscle of the shells ; i, anterior retractor muscle of the 

 foot ; fcr protractor muscle of the foot ; I, posterior (pedal) adductor muscle 

 of the shells ; m, posterior retractor muscle of the foot ; n, anterior labial 

 tentacle ; o, posterior labial tentacle ; p, base-line of origin of the reflected 

 mantle-flap from the side of the body ; g, left external gill-plate ; r, left in- 

 ternal gill-plate ; tt, inner lamella of the right inner gill-plate ; rg, right outer 

 gill-plate ;s, line of concrescence of the outer lamella of the left outer gill- 

 plate with the left mantle-flap ; t, pallial tentacles ; u, the thickened mus- 

 cular paUial margin which adheres to the shell and forms the pallial line of 

 the left side ; v, that of the right side ; w, the mouth ; x, aperture of the left 

 organ of Bojanus (nephridium) exposed by ciitting the attachment of the 

 inner lamella of the inner gill-plate ; y, aperture of the genital duct ; 2, flssure 

 between the free edge of the Inner lamella of the inner gill-plate and the side 

 of the foot, through which the probe g passes into the upper division of the 

 sub-pallial space ; oa, line of concrescence of the inner lamella of the right 

 inner gill-pme with the inner lamella of the left inner gill-plate ; a&, ck, ad, 

 three pit-like depressions in the median line of the foot supposed by some 

 writers to be pores admitting water into the vascular system ; ae, left shell 

 valve ; af, space occupied by liver ; ag, space occupied by gonad ; ah, muscular 

 substance of the foot ; ai, duct of the liver on the wall of the stomach ; ale, 

 stomach ; al, rectum traversing the ventricle of the heart ; am, pericardium ; 

 an, glandular portion of the left nephridium ; ap, ventricle of the heart ; aq, 

 apei'ture by which the left auricle joined the ventricle ; ar, non -glandular por- 

 tion of the left nephridium ; as, anus ; at, pore leading from the pericardium into 

 the glandular sac of the left nephridium ; om, pore leading from the glandular 

 into the non-glandular portion of the left nephridium ; av, internal pore lead- 

 ing from the non-glandular portion of the left nephridium to the external 

 pore X ; aw, left oerebro-pleuro-vlsceral ganglion ; ax, left pedal ganglion ; 

 ay, left otocyst ; or, left olfactory ganglion (parieto-splanohnic) ; 66, floor of 

 the pericardium separating that space from tlie non-glandular portion of the 

 nephridia. 



The free edge of the left half of the mantle-skirt b is repre- 

 sented as a little contracted in order to show the exactly simi- 

 lar free edge of the right half of the mantle-skirt c. These 

 edges are not attached to, although they touch, one another; 

 each flap (right or left) can be freely thrown back in the way 

 which has been carried out in fig. 124, (3) for that of the 

 left side. This is not always the case with Lamellibranchs; 

 there is in the group a tendency for the corresponding 

 edges of the mantle-skirt to fuse together by concrescence, 



and so to form a more or less completely closed bag, as in 

 the Scaphopoda (Dentalium). In this way the notches 

 d, e of the hinder part of the mantle-skirt of Anodon are in 

 the Siphonate forms converted into two separate holes, the 

 edges of the mantle being elsewhere fused together along 

 this hinder margin. Further than this, the part of the 

 mantle-skirt bounding the two holes is frequently drawn out 

 so as to form a pair of tubes which project from the shell (figs. 

 130, 141). In such Lamellibranchs as the oysters, scallops, 

 and many others which have the edges of the mantle-skirt 

 quite free, there are numerous tentacles upon those edges. 

 In Anodon these pallial tentacles are confined to a small area 

 surrounding the inferior siphonal notch (fig. 124, (3), t). 



The centro-dorsal point a of the animal of Anodonta 

 (fig. 124, (1)) is called the umbonal area ; the great anterior 

 muscular surface h is that of the anterior adductor muscle, 

 the posterior similar surface i is that of the posterior 

 adductor muscle ; the long line of attachment u is the 

 simple " pallial muscle,"' — a thickened ridge which is seen 

 to run parallel to the margin of the mantle-skirt in this 

 Lamellibranch. In some of the Siphonate Isomya, which 

 are hence termed " Sinupallia," the pallial muscle is not 

 simple but deeply incurved at the posterior region so as to 

 allow of the large pallial siphons being retracted within the 

 shell or expanded at wUl (fig. 127, and figs. 140, 141). 



It is the approximate equality 

 in the size of the anterior and 

 posterior adductor muscles which 

 has led to the name Isoyma for 

 the group to which Anodon be- 

 longs. The hinder adductor 

 muscle may be considered as re- 

 presenting morphologically the , 

 transverse fibres of the root of | 

 the foot of Nautilus by which it 

 adheres to its shell (fig. 91, A), the 

 annular muscular area of Patella 

 (fig.27,c), and the columella muscle 

 of the Gastropods generally. It 

 is always large in Lamellibranchs, 

 but the anterior adductor may 

 be very small (Heteromya), or ^''^il^s.-Viow of the two valves 



, , •' , , , , \ /■»«-' \ of the shell of Oytherea (one of 

 absent altogether (Monomya). the Sinupalliate isomya), from 



The anterior adductor muscle is the dorsal aspect, 

 in front of the mouth and alimentary tract altogether, 

 and must be regarded as a special and peculiar deve- 

 lopment of the median anterior part of the mantle -flap 



(-'■'-'■■ limale 



width 



ligament 



^saljl^^^^o/^^^ 



umto 



Jx luitult. 



Fjg. 126.— Right valve of the same shell from the outer face. 



in Heteromya and Isomya. Amongst those Lamelli- 

 branchs which have only a posterior adductor (Monomya), 

 it is remarkable that the oyster has been found (by 

 Huxley) to possess, when the young shells and muscles 

 first develop, a well-marked anterior adductor as well as a 

 posterior one. Accordingly there is ground for supposing 



