202 THE LAMELLIBRANCHIA 
stage (Fig. 192, B). In the third or deutomonomyarian stage, the 
anterior adductor has disappeared (Fig. 192, C). 
The two adductors, by their contraction, bring together the 
valves and close the shell. They are diminished in size when the 
valves lose their mobility, as, for example, in Galeomma, Ephippodonta, 
and Scioberetia, and they actually disappear in <Aspergillum and 
Chlamydoconcha. These muscles are generally perpendicular to the 
surfaces of the valves, but in some Lamellibranchia that are fixed 
by one side, they may be very oblique, eg. Anomia, Pecten, the 
Rudistae. The fibres of the adductors 
are attached to epithelial cells of the 
mantle which produce the hypostracum 
or substance of the muscular impres- 
sions. In many cases the fibres of 
each adductor may be divided into 
two distinct parts of different appear- 
ance (Pecten, Ostraea, ete.), of which 
the principal part in the monomyarian 
Lamellibranchs is formed of apparently 
striated fibres (see p. 5), this appar- 
ent striation being particularly well 
defined in the muscles of the swim- 
ming Lamellibranchs, Pectinidae, and 
Limidae, which are capable of very 
rapid contraction. The absolute force 
exerted by the adductor muscles is 
analogous to that of vertebrate 
muscles: in some cases they can resist 
the traction of a weight equal to 
several thousand times the weight of 
the animal deprived of its shell. 
In the siphonate Lamellibranchia 
the pallial fusions separating the 
branchial and pedal orifices are often 
furnished with crossed muscular 
bundles, called cruciform muscles, 
passing from the edge of one valve 
to that of the other, and thus forming 
accessory adductor strands: these may 
Heche ie be seen in JZellina, Syndosmya, and 
Solenocurtus strigillatus, ventral as- Dona among the Tellinacea, and in 
pect. a.s, anal siphon; br.s, branchial o | ? i 
siphon ; f, foot; mu, cruciform pallial Solenocurtus (Fig. 194, mu). In species 
muscles; pa, mantle; pa.c, pallial “ ee + 
cavity: a shells) (Aten Pali.) called “closed,” in which the mantle 
fusions are very extensive, these muscles 
are continuous along the whole ventral border of the valves, between 
the pedal and branchial orifices (Saxicava, Fig. 246, pw). 
