15 



The vputial body-wall in the head region is lliioker, 

 but possesses almost solely trausverse and oblique fibres. 

 The muscle of this region is continuous witli that of the 

 toot and shell muscle. 



The general appearance of the foot has already been 

 described. It is essentially a tliichening of tlie vential 

 body Avail made iip, for the most part, of bundles of 

 muscle fibres bound together by connective tissue. In 

 the ventral ])art the fibres run in all directions, while in 

 the dorsal pai't they are mostly horizontal and oblique. 

 The fibres in the outer ])ortion of the foot are steeply 

 oblique, and these, together with some of the dorsal 

 fibres, are continuous with the shell muscle. The 

 absence of a horizontal circular muscle, and the many 

 differences between the disposition of fibres here and in 

 the suckers of various animals, tell against the hypothesis 

 of a sucker-like action of the foot, which, as was said 

 above, is also not supported by observation. The spaces 

 between the bundles of muscle fibres contain blood and, 

 when full, make the foot swell far beyond its usual size. 

 Two of the blood spaces above mentioned are of special 

 importance, the}' run along in the vicinity of the pedal 

 nerve-cords, and it is from them that blood s]>reads to 

 the remainder of the organ (fig. Sa). 



In very young specimens (about | in. long) there is a 

 distinct glandular outgrowth from either side of the foot, 

 which disappears more or less completely when the animal 

 gets older. The outgrowth is, from its position and 

 histological characters, the homologue of the " lateral 

 streak" in Nacella, etc. There is also a rudimentary 

 flap covering the glandular tissue in the anterior 

 region, where also the whole structure is best developed. 

 The persistence of this glandular' tissue is possibly con- 

 nected with the orotection of the vouno" animal against 



