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 
Fic. 191. 
Right-side view of a Mactra, the right valve 
of the shell and right mantle-flap removed, and 
the siphons retracted. br, b7’, outer and inner 
gill-plates ; ec, umbo of the shell; m.a, anterior 
adductor muscle; m.p, posterior adductor 
muscle; m.s, pallial retractor muscle of the 
siphons; p, foot; ¢, labial palp; ta, 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 
Fig. 192. 
Three stages in the development of Ostraea, right-side view. A, protomonomyarian stage of 
O. edulis (atter Woodward) ; B, dimyarian stage ; and C, deutomonomyarian stage of O. virginiana 
(after Jackson). a, anus; a.@, anterior adductor ; ¢.g, cerebral ganglion; f, foot; g, gill; in, 
intestine ; /./, left liver; /.p, labial palp; m, mouth ; oe, oesophagus ; p.a, posterior adductor ; 
p.e, pallial edge; p.g, pedal ganglion; pr, prodissoconch ; 7./, 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, Modiolaria, Pecten, Ostraea (Fig. 192, A), Dreissensia 
(Fig. 224, a.a), Unionidae (Fig. 227, 2), Pisidiwm, Montacuta, Lasaea, 
Entovalva, Cardiwm, Pseudokellya, etc.), but diminishes in importance 
