No. 3.] THE MUSCULATURE OF CHITON. 597 



are flattened against each other, or even overlap, in which case 

 the natural position seems to be with the posterior end curled 

 under the anterior ; if the anterior edge of the mantle folds 

 under the posterior, the animal may several times open and 

 close, the posterior edge getting more and more nearly under 

 the anterior at each attempt, until finally the posterior edge is 

 entirely folded under, and the animal remains quiet. It lies 

 thus rolled together for some time on the bottom of the dish 

 or on the stone ; in trying to recover its footing, it is able to 

 arch the ventral surface of the foot, thus making its dorsal side 

 bearing the shells concave, to such an extent that the ventral 

 surface of both anterior and posterior edges of the mantle is 

 turned under, and rests on the stone or other surface on which 

 the animal has been lying ; this position is not retained very 

 long at a time, but is repeatedly assumed after intervals of rest 

 in the contracted state. Such attempts to regain the normal 

 position do not appear to be successful unless by its exertions 

 the animal may happen to move to a part of the stone where it 

 can lay hold of some irregularity of the surface. The Chiton, 

 when on its back, can twist itself into most contorted shapes, 

 and may finally attach itself to a stone if the stone lies 

 near by. 



Muscles of the Shells. 



Changes in the relative positions of the shells are brought 

 about by muscles immediately ventral to the shells, which can 

 be removed in somewhat imperfectly preserved specimens. 

 The muscles are then seen exposed on the dorsal surface 

 (Fig. 2). 



They are: (i) A median dorsal longitudinal muscle (Figs. 2, 



3, also 4, 5, 10, md), whose fibres pass for the most part from 

 the anterior edge of each shell, between the apophyses, to the 

 ventral surface of the anterior part of the next anterior shell, 

 though some fibres are continuous under the junctions of the 

 shells ; this muscle is thickest on either side of the middle 

 line, except at the point of the posterior attachment {cf. Fig. 



4, md with Fig. 5, nid). 



