136 ANIMAL — INTERNAL ORGANIZATION. 
and primitively ami usually contains the respiratory organs, and the 
various organic apertures, its thickened glandular margin secreting 
the calcareous and chitinous deposits forming the shell, although the 
shell-secreting function is not confined to this part, as the M'hole 
surface of the mantle is directly concerned in the formation of the 
internal calcareous layers of the shell, the animal being attached 
thereto hy powerful muscular hands, which are symmetrically paired 
or unicpie according to the group : and finally 
The Body or Visceral region, which is generally developed in a 
more or less protuberant form and placed above the foot and partially 
or comiiletely covered or enclosed hy the i)allium. It contains the 
organs of reproduction, the heart or motive centre of the vascular sys- 
tem, the alimentary canal, and various secretory and excretory organs. 
Internally the general unity and agreement of the jdan of 
organization, though it may have become more or less obscured hy 
later develupments and modihcations, is much more evident. 
Beneath the eiiithelium, the name given to tissue covering a free 
.space, connective tissue of mesodermic origin is found, comjmsed 
chiefly of vesicular and 2)lasm cells, 
so closely interwoven with the 
subcutaneous musculature as to 
form a kind of dermo- muscular 
tube, to which greater firmness and 
rigidify is somefinies imparted by 
numerous calcareous concretions, 
produced hy, aud lodged within, 
its ve.sicular cellules. This inter- 
laced subcutaneous stratum, some- 
times called the corium, at times 
attains a great development, and 
gives rise to outgrowths of various 
kinds, which are liable to undergo 
concrescence or fusion amongst themselves, or with other organs of 
the body. The coelomic cavity contains the various organs of the 
body ami a great develoi)ment of connective tissue, which is i)er- 
meated in all directions hy numerous lueniatocades or hlood-siiaces, 
the distension of which hy the circulatory fluid is the cause of the 
turgescence or enlargement of the different organs. This tissue 
originates in the form of polyhedral nucleated cells of homogeneous 
Fig. 21H. -Typical Connective Tissue, rich in 
plasm cells and shouin.;? interlacing fihrillar 
bundles and small blood lacunai, from IJclix 
nciuoralis^ highly magnified (after Brock). 
