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powerful musculature, resist the pressure of the water within the 

 shell, and compel it to escape anteriorly and posteriorly quite near 

 the hinge line. The muscles that raise the velum to this position 

 are those that appear so distinctly striped in sections (Fig. 1, Striped 

 Mus.). 



They arise near the shell side of the mantle at the base of the 

 periostracal groove (Fig. 1, Per. G) and pass across to the base of 

 the velum. They lie almost at right angles to the plane of the mantle 

 and on the distal side of the circumpallial nerve (Fig. 1, -A^.). They 

 pass into the velum for some distance, becoming more confined to the 

 outer surface. The bulk of the muscles in the velum run parallel to 

 the mantle edge and are seen therefore in cross section in Fig. 1. 

 They serve to keep the velum in position whilst the valves are being 

 closed. 



The striped muscle fibres do not form a regular sheet extending 

 along the mantle edge, but occur in bundles (Fig. 2, Striped Mus.)., 



.-, opif 



striped 

 Muscle 



Fig. 1. 



Fig. 2. 



Fig. 1. Transverse Section. Mantle edge of Pecten jacobaeus. X 15. 

 Fig. 2. Section. Mantle edge of P. jacobaeus. X ^^• 



between the bases of the eye stalks. This can be seen in Fig. 2 

 which represents a section along the line AB in Fig. 1. Wherever 

 tliere is an eye, there is a development of more compact connective 

 tissue with fewer muscles so that the course of the optic nerve from 

 the circumpallial nerve to the sense organ is unimpeded. Between 

 these points the striped muscles occur. Fol (3) and more recently 

 Marceau (6) have discussed the question of the double oblique striation 

 in the muscle fibres of Lamellibranchs and have shown that it is an 

 optical effect due to a series of fine fibrillae which run spirally around 

 the fibres in one direction only. 



Marceau finds that these fibrillae are sometimes not homogenous 

 but have a regular alternation of dark and light elements. These ele- 



