PATELLA. 209 



are numerous blood spaces, the most important pair of 

 which have already been mentioned. It is the quantity 

 of blood in these spaces that governs the turgidity of the 

 foot and therefore its functional activity. Besides the 

 above muscle fibres there are others which run horizontally 

 and obliquely especially in the ventral part. The muscle 

 bundles are wrapped in connective tissue. 



The lateral glandular streak is a much larger projection 

 than the pucker ings of the side surface of the foot and it 

 occurs amongst these just below the ventral limit of the 

 mantle cavity, extending backwards a variable distance 

 on each side. It consists mainly of tubular gland tissue, 

 the upper surface of which is covered by small columnar 

 epithelium as also is the under surface except where the 

 glands open. There is very little connective tissue 

 between the glandular tubes. The epidermal and sub- 

 epidermal tissue just ventral to the projection is to some 

 exitent vacuolated. A variable number of muscle fibres 

 go out to the projection, and dorsal to it, in the anterior 

 region, is found a vestige of a flap formed by muscle fibres 

 and connective tissue covered by epithelium. 



The muscles are composed of smooth fibres. These may 

 be scattered, when they are seen to be each an elongated 

 cell with lateral nucleus, or aggregated into bundles, as 

 in the shell muscle. None of the fibres of this muscular 

 tissue show striation. 



The Shell Muscle. — The bundles of oblique fibres 

 forming the dorsal part of the foot are continued around 

 its dorsal rim into the shell muscle', so called because its 

 fibres are attached to the shell (by means of a compressed 

 epidermis). This muscle encircles the visceral hump 

 except at the anterior end where it is wanting across the 

 neck region, it is therefore of a horse-shoe shape and the 

 anteriorly placed ends of the horse-shoe are enlarged. 

 s 



