No. 3-] THE MUSCULATURE OF CHITON. 605 



ber, and the attachments of the muscles are accordingly equally 

 limited posteriorly ; the branchial artery sends a branch toward 

 the body cavity in the same region (Fig. 13, VII, attachments 

 of the anterior group). 



The nephridial duct and extended auricle cut off also the 

 anterior portion of the attachment of the latero-pedal and 

 antero-oblique muscles of the posterior group, until the anterior 

 limits of these attachments are relatively posterior to the an- 

 terior limit of the medio-pedal (Fig. 13, VII, posterior group). 

 The postero- and two antero-oblique muscles thread their way 

 between the various ducts and blood vessels as strands rather 

 than bands of muscles, as under the other shells. 



Thus under VII not only is the space between the muscular 

 attachments occupied by organs other than the nephridial 

 branches, but the attachments even of the muscles to the shells 

 are reduced, and the oblique muscles are compressed into small 

 spaces between the ducts and vessels ; notwithstanding these 

 modifications, however, muscles corresponding to all of those 

 under the typical shell can be clearly recognized, and have a 

 perfectly typical distribution. 



Shell VIII. 



Under VIII the postero-oblique muscle is merely a small 

 group of fibres, not entirely wanting, as I at first supposed. 

 The antero-oblique fibres, on the other hand, are especially 

 prominent. There is no sharp division of the muscles into 

 two groups, but the latero- and medio-pedal fibres form almost 

 continuous muscles from the anterior to the posterior limits of 

 their attachments : both muscles begin on the ventral surface 

 of the most anterior portion of the apophysis, a point which is 

 relatively in front of the usual attachment even of the medio- 

 pedal, under the other shells ^ (Fig. 13, VIII). Toward the 

 posterior part of shell VIII the muscles of the opposite sides 

 gradually approach (corresponding to the narrowing of the 



1 In a preceding account, a sufficiently anterior limit was not attributed to 

 these muscles. It should also be noted that in the diagram, Fig. 2 of the account 

 referred to, the median dorsal muscle of shell II is extended too far anteriorly 

 k^Jenaische Zeitsch. f. Naturwiss. xxviii, Bd. N.F. xxi). 



