19 



lie tlie o>ills (fig. 1, Br. d., Br. a.) and the lower part of 

 the visceral mass. 



The Mantle consists of two tliin lobes, folds of the 

 tegumentary layer of the body, with epithelium coverings 

 both externa! and internal surfaces (fig. 4, E. Mv.), and 

 but little connective tissue and muscle fibres except at 

 the free marg-in which is much thickened and muscular. 

 The mantle epithelium is the outermost layer of the body, 

 the shell being- a secretion on its surface. The outer 

 layer which lines the shell extends from the hinge line 

 (where it becomes continuous with the same layer on the 

 other side) to the ventral edge of the mantle, as a con- 

 tinuous sheet. It is to be found, though modified, 

 between the adductor muscle and the shell, lying between 

 the miisele fibres proper and the peculiar calcified layer 

 (fig. 2, Sh. »i.) which is found on the internal surface of 

 the shell at the muscle impressions. 



The inner layer is reflected inwards at several points 

 to be continued over the visceral mass. For example, it 

 passes over the adductor muscle and on to the gonad ; 

 dorso-posteriorly it runs across from one mantle fold to 

 the other just above the pericardium (fig. 1, Per.), partly 

 forming its roof and supporting the posterior pallial 

 artery (fig. 14, A. jj. p.) which can be easily seen running 

 up towards the hinge line. The two layers of the mantle 

 do not pass over the sides of the digestive gland (fig. 1, 

 /^f/.). The inner one becomes closely apposed to it, 

 anteriorly and posteriorly, forming the body wall here, 

 whilst the outer epithelial layer alone clothes the sides of 

 the gland. 



Dorsally the right and left mantle folds are con- 

 tinuous along the full length of the hinge line, as has 

 already been pointed out, but the level of this is broken 

 about tlic middle of i1s leno-th where there is a rectano-ular 



