No. 3-] THE MUSCULATURE OF CHITON. 615 



Shell I. 



Under I the median dorsal muscle is separated into two 

 diverging parts, each oblique dorsal likewise into two. The 

 lateral longitudinal passes from I posteriorly to II, but is not 

 extended anteriorly from I, because of the terminal position of 

 the shell; an oblique muscle from the posterior part of the 

 shell passes with a parallel oblique muscle from II into the foot. 

 A system of dorso-ventral fibres in the head-fold is separated 

 by horizontal buccal muscles into two small anterior groups, 

 and a posterior group in which can be recognized representa- 

 tives of the latero-pedal muscles of the anterior and posterior 

 groups of other shells. The muscles of the two sides gradually 

 approach as the anterior end of the body becomes narrower; 

 fibres corresponding to the medio-pedal muscles of other shells 

 are attached to a lateral region of the shell in the mantle. The 

 interior mantle muscle is interrupted by two pairs of horizontal 

 buccal muscles, and its attachment partially interrupted by a 

 median muscle to the lip. 



In the anterior lip is also a median horizontal muscle at- 

 tached to I; circular muscles occur about the mouth, and 

 longitudinal muscles from before and behind radiate into the 

 lips. 



Muscles of the Radula. 



To the description of the other muscles of Chiton may be 

 added a very brief sketch of the muscles of the radula, and of 

 their general direction and places of attachment. 



The relations of the radula to the mouth and other organs 

 are best seen in longitudinal sections {cf. Fig. 8) ; the mouth 

 opens into the pharynx, which leads dorsally into the oesopha- 

 gus (Fig. 8, oe)\ ventral to the oesophagus, the radula sheath 

 opens into the pharynx (at r. Fig. 8), and fits around the radula 

 or lingual ribbon, the radula and sheath being extended pos- 

 teriorly for a long distance, ventral to the oesophagus. On 

 either side of the radula are the so-called "cartilages," long 

 hollow vesicles with thick cartilaginous walls which serve for 

 the attachment of muscles; anteriorly, the radula sheath is 



