No. 3-] THE MUSCULATURE OF CHITON. 6oi 



opposed to the action of the sagittal and horizontal cushion 

 muscles (which draw the posterior part of the dorsal shell 

 nearer to the ventral) in the extension of the animal. 



Muscles of the Foot. 



The muscles to the foot are attached to the ventral surface 

 of the shells, on either side, between the body cavity and the 

 mantle, whence they pass in various directions into the foot 

 and are dispersed through the whole organ; some of the fibres 

 cross the middle line, and the body cavity is bounded by muscle 

 fibres below and at the sides (Figs. 4, 5). 



Corresponding with the regularity in the relations of the 

 shells, the muscles that occur under one shell are exactly re- 

 peated under each of the others, except where the regularity is 

 interrupted by other organs. For any cross-section of shell IV, 

 with the underlying muscles, there is an exactly corresponding 

 section of shells V and VI. The same order of muscles is 

 easily recognized under III and also under VII, where, how- 

 ever, the connections between the auricles and the branchial 

 veins, and the efferent ducts of the nephridia and reproductive 

 organs introduce modifications; I and VIII are different, owing 

 to their terminal positions, and the buccal muscles under II 

 (and also under I) disguise to some extent the usual order, 

 although, even in these cases, muscles corresponding to those 

 under the other shells occur. 



Shell VI. 



I shall first describe the muscles under a typical shell — 

 for example under VI — and shall limit the description to the 

 muscles of one side of the animal (the bilateral symmetry being 

 perfect). 



The muscles are divided into two groups, which, near the 

 shell, are separated from each other {cf. under V, Figs. 6, 2, 

 right side). Owing, however, to the diverse directions in which 

 the fibres run, the muscles form at a lower level a compact 

 mass from end to end of the animal, completely occupying the 

 foot and the narrow space between the mantle chamber and 

 body cavity. 



