PATELLA. 207 



The ventral body-wall in the head region is thicker, 

 but possesses almost solely transverse and oblique fibres. 

 The muscle of this region is continuous with that of the 



foot and shell muscle 





The general appearance of the foot has already been 

 described. It is essentially a thickening of the ventral 

 body wall made up, for the most part, of bundles of 

 muscle fibres bound together by connective tissue. In 

 the ventral part the fibres run in all directions, while in 

 the dorsal part they are mostly horizontal and oblique. 

 The fibres in the outer portion 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:, they run along in the vicinity of the pedal 

 nerve-cords, and it is from them that blood spreads to 

 the remainder of the organ (fig. 8a). 



In very young specimens (about J- 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 protection of the young animal against 



