No. 3-] THE MUSCULATURE OF CHITON. 609 



Shell I. 



To understand the distribution of the muscles under I, it is 

 necessary to bear in mind that ventral to it the radula, with its 

 enormous supply of muscles, occupies the cavity of the head 

 region ; that large bundles of buccal muscles pass from the 

 shell ; that the mouth and lips occupy a portion of the ventral 

 surface ; and that the oesophageal ring (giving off the branchio- 

 visceral and pedal branches) occurs in this region ; also, that 

 the head cavity gradually diminishes in width, toward its 

 anterior end, and that the line on which the muscles are 

 attached is not straight, but curved, i.e., parallel to the anterior 

 edge of the mantle or shell {cf. Fig. 3). To the posterior part 

 of I is attached a "dorso-ventral" muscle immediately anterior 

 to II ; portions of its fibres pass either side of the oesophageal 

 ring and of the oblique muscle to the foot (Fig. 7, dv), and are 

 distributed to the lateral portions of the head-fold and in part 

 to the lips, corresponding to the latero-pedal fibres of the 

 posterior group of the other shells. Slightly anterior to these, 

 is another group of "dorso-ventral" fibres, that passes entirely 

 outside the oesophageal ring: these fibres may be referred to 

 the latero-pedal fibres of the anterior group, although no sharp 

 division exists between them and the more posterior fibres. 

 The more anterior dorso-ventral fibres are attached to the shell 

 with fibres of the oblique dorsal muscle {cf. Fig. 3), just as 

 the latero-pedal of the anterior group under the other shells is 

 connected with the anterior attachment of the most lateral 

 oblique dorsal fibres. I have called these groups of fibres the 

 dorso-ventral muscles, as a matter of convenience, although 

 their direction is rather oblique than dorso-ventral ; the muscles 

 in the region of the buccal mass are crowded to a relatively 

 more lateral attachment to the shell than is the case with the 

 corresponding muscles under other shells. Fibres that can be 

 roughly compared to the medio-pedal muscles (" horizontal 

 fibres," Fig. 7, h) are attached to the lateral portion of the 

 shell, far out in the mantle, the fibres lying almost in the 

 horizontal plane ; this position is possible in the region of the 

 body in which the fibres occur, for the gills, with their nerve 



