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SEA-FISIERIES LABORATORY. veer k 
lie the gills (fig. 1, Br. d., Br. a.) and the lower part of 
the visceral mass. } 
The Mantle consists of two thin lobes, folds of the 
tegumentary layer of the body, with epithelium covering 
both externai and internal surfaces (fig. 4, H. Mn.), and 
but little connective tissue and muscle fibres except at 
the free margin 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 muscle fibres proper and the peculiar calcified layer 
(fig. 2, Sh. m.) 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. p. 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, 
Dg.). 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 the middle of its length where there is a rectangular 
