212 



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, Ephippod&nta, 

 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, e.g. 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, etc.), 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 

 be seen in Tellina, Syndosmya, and 



0.4 



Fio. 194. 



soknocurtus strigiiiatu*, ventral as- J) jiax among the Tcllinacca, and in 



pert, a.*, anal siphon ; br.s, branchial "L 



siphon;/, foot; mu, cruciform pallial boienOCUrtUS (-tig. 194, mU). In SpCClCS 



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, pa). 



