124 COMPARATIVE ANATOMY CHAP. 



base of the foot these two bands unite to form a single muscle on each 

 side, which enters the foot and radiates out through it in the form of 

 numerous longitudinal bundles. This then is a paired eolumellar 

 muscle which retracts the foot, and draws the whole of the lower 

 portion of the body back into the upper part of the shell. 



D. Lamellibranehia. 



The two principal groups of muscles to be considered in this 

 class are : 



1. The pallial musculature. 



2. The pedal musculature. 



The former is principally developed near the free edge of the 

 mantle, and consists of three systems: (1) Fibres which run in the 

 plane of the mantle fold towards and at right angles to its edge. 

 These are, in the narrower sense, the muscles of the pallial edge, 

 and leave on the shell the scar known as the pallial line. (2) 

 Fibres running parallel with the edge of the mantle. (3) Short trans- 

 verse fibres running more or less straight between the inner and the 

 outer surfaces of the mantle. In the siphons, which are formed from 

 the mantle, these three systems become circular, longitudinal, and 

 radial layers. The retractors of the siphons are a special differentia- 

 tion of the pallial musculature ; their development is in direct relation 

 to the size of the siphons ; their crests of attachment to the shell 

 valves cause the scar known as the pallial sinus (cf. p. 64). 



The important adductor muscles for closing the shell must 

 also be regarded as differentiations of the pallial musculature. These 

 are exceedingly thick and powerful and run transversely from the 

 inner surface of one valve to the corresponding surface of the other 

 valve. They counteract the ligament at the hinge, their contraction 

 causing the two valves to approach one another, till the shell is closed. 

 These adductors leave scars on the inner surfaces of the valves. 

 Typically, there are two adductors, an anterior and a posterior 

 (Dimyaria), situated nearer the dorsal than the ventral edge of the 

 valves. In the Myttiacta, the posterior adductor is larger than the 

 anterior (Heteromyaria as opposed to Isomyaria). In one large 

 series of forms the anterior adductor completely atrophies, and the 

 posterior adductor, which is all the more strongly developed, shifts 

 forwards towards the middle of the shell. These forms are grouped 

 together as Monomyaria ; but this is no natural group, since nearly- 

 related forms (e.g. within the Muellenacea) may possess either one or 

 two adductors, and widely different forms (e.g. Tridacna, Anomia, 

 MneUeria, Asperyillum) agree in having only one. The Anomiidce, 

 Ostreidce, Spondylidce, Limidce, Pectinidce, Aviculidce, Muelleridce, etc., are 

 Monomyarian. 



The adductor often (e.g. Peden, Ostrca, Niicula) consists of two apparently 

 different parts, one containing smooth fibres and the other fibres which appear 



