210 



THE LAMELLIBRANCHIA 



the shell and as a retractor of the pallial border. (2) At the 

 posterior end a specialised part of the orbicular muscle originates 

 from and serves as the retractor of the siphons (Fig. 191, m.s) its 

 development is proportional to that of the siphons, and it interrupts 

 the evenly curved line formed by the orbicular muscle. The 



Fio. 191. 



Right-side view of a Mactra, the right valve 

 of the shell and right mantle-flap removed, and 

 the siphons retracted, br, W, outer and inner 

 gill-plates ; c, umbo of the shell ; m.a, anterior 

 adductor muscle ; m.p, posterior adductor 

 muscle ; m.s, pallial retractor muscle of the 

 siphons; p, foot; t, labial palp; to, anal 

 siphon ; tr, branchial siphon. (From Lan- 

 kester, after Gegenbaur.) 



muscles leave an " impression " on the inner surface of each valve 

 of the shell, and the impression of the orbicular muscle or " pallial 

 impression" (Fig. 189) is indented by a "sinus" formed by the 

 siphonal muscles. This " sinus " is scarcely ever absent in siphonate 

 forms except in the Lucinidae and in Cuspidaria, in which the 

 siphons are not retractile, and in the latter genus are protected by 

 prolongations of the posterior part of the shell. (3) The adductor 

 muscles of the shell are transverse pallial muscles uniting the two 

 lobes of the mantle and the two shell-valves secreted by them. 

 There are at the most two adductors ; the anterior is dorsal and in 



Three stages in the development of Ostraea, right-side view. A, protomonomyarian stage of 

 0. edulis (after Woodward) ; B, dimyarian stage ; and C, deutomonomy arian stage of 0. virginiana 

 (after Jackson), a, anus ; u.a, anterior adductor ; c.g, cerebral ganglion ; /, foot ; g, gill ; in, 

 intestine ; l.l, left liver; l.p, labial palp; m, month ; oe, oesophagus ; p.a, posterior adductor ; 

 p.e, pallial edge ; p.g, pedul ganglion ; pr, prodissoconch ; r.l, right-liver lobe ; sh, shell ; st, 

 stomach ; ve, velum ; v.g, visceral ganglion. 



front of the buccal aperture (Figs. 188, a.a; 191, m.a), but extends 

 considerably towards the ventral side in Modiolaria and Chama. 

 The anterior adductor muscle is the first to appear in development 

 (Nucula, Mytilus, Modiolaiia, Pecten, Ostraea (Fig. 192, A), Dreissensia 

 (Fig. 224, rt.), Unionidae (Fig. 227, z), Pisidium, Montacuta, Lasaea, 

 Entovalva, Cardium, Pseudokellya, etc.), but diminishes in importance 



