No. 3-] THE MUSCULATURE OF CHITON. 613 



between each shell and the apophysis of the shell next poste- 

 rior, composed of oblique fibres in the sagittal plane, and, 

 under the posterior part of these, oblique fibres in the horizon- 

 tal plane, both directed so that the more anterior attachment 

 is that to the ventral shell ; and also oblique fibres in the 

 transverse plane at the antero-lateral edge of the apophysis, 

 continuous with the fibres of a large mantle muscle where the 

 apophysis is of sufficient width to reach the mantle. (2) Two 

 groups of muscles to the foot, separated from each other near 

 the shell by a space occupied by nephridial branches ; each 

 group consists of an inner latero-pedal, a middle antero-oblique, 

 and an outer medio-pedal muscle that crosses the latero-pedal, 

 and continues ventrally the boundary of the body cavity (each 

 with a distribution denoted by its name); a postero-oblique 

 muscle occurs in the anterior group, and fibres in the ventral 

 part of the foot, not united into defined muscles, complete the 

 supply to the foot. (3) In the mantle, a muscle from the part 

 of the ventral surface of the shells that is immediately dorsal 

 to the mantle chamber, interrupted at the shell for a short dis- 

 tance anterior to the region of the anterior group of pedal 

 muscles, and interrupted again more extensively anterior to the 

 region of the posterior group ; fibres from the extreme edges 

 of the shell to the mantle, and scattered fibres of various direc- 

 tions in the ventral part of the mantle. The deviations from 

 the typical arrangement under the other shells will be noted. 



Shell III. 



The antero-oblique muscles are reduced and the postero- 

 oblique is enlarged. 



Shell VII. 



Under VII the arrangement of the muscles is modified by 

 the efferent ducts of the internal organs, and by the blood 

 vessels that here occur. The attachment of the medio-pedal of 

 the anterior group is limited anteriorly and posteriorly, that 

 of the postero-oblique is limited posteriorly and the attachment 

 of the antero-oblique is slightly cut off posteriorly : the ante- 



