18$15.] Vascular and Extra Vascular Parts of Animals, 177 
they resemble the exuvial or epidermoid membranes. To these 
facts may be added the notorious circumstance of the unchangeable- 
ness of the outer surfates of testaceous shells during their growth, 
and the continual renewal of their other surfaces, which admit of 
contact with the living inhabitant: next the stains and coloured 
transudations which they often derive from metallic salts and other 
colouring materials placed in their vicinity: and, lastly, that such 
occurrences do not affect the living animal. The mechanical ‘con- 
nexion or contact that subsists between the living animals which 
occupy the testaceous shells, and their extraneous dwellings are in 
many instances very slender. The common oyster possesses its first 
pair of valves, consisting of single laminz, before it quits the 
parental organs. A muscle passes between the centres of the cavity 
of each shell adhering to each, and it acts upon the valves nearly at 
right angles. ‘The animal has no other continuity with the shells. 
At the hinge an elastic substance is wedged in, the spring of which 
is excited by compression, but it does not possess the property of 
extension beyond its passive state ; when dried, this substance cracks 
into cubes. As the animal grows, it augments the margin of its 
shells, and thickens them by adding new laminz on their insides. 
The muscular adhesion glides forward, still keeping to the centre of 
the valves. ‘The elastic substance at the hinge is augmented along 
its inner surface only, and must have been always deposited during 
the expanded state of the valves, since the limit of its elastic con- 
dition is exactly adapted to that state. As the lamin:e of the shells 
increase, there is a gap at the outside of the hinge filled with soft, 
crumbling, and decomposing worn-out elastic ligament. This ga 
presents two inclined planes which meet in an acute angle, and that 
space is kept free from pebbles and hard extraneous bodies by the 
presence of the decomposing ligament, as such an accident would 
prove fatal by preventing the opening of the valves. The growth 
of all the testaceous shells affords remarkable proofs of their extra 
vascular formation. ‘The muscular adhesions are generally the only 
parts of continuity between the animal and its shells, and these are 
constantly changing with the augmentations of bulk. In all the 
conoid univalves which revolve upon spiral axes the successive parts 
of the shell are merely spread upon the older parts without any in- 
termixture of their substances, and epidermis or extraneous bodies 
are alike involved in the successive folds. In other classes of animals 
similar phenomena occur. The calcareous shells of birds’ eggs are 
merely deposited upon the membrana putaminis, and the inner por- 
tions are regular crystallized prisms, the long diameters of which 
point to the centre of the egg. These shells are wholly extra vas- 
cular, and their albuminous membranes are alike cuticular, whilst 
the inner trae membrana putaminis is made reticular, and capable 
of vascular organization. The order of deposit in these examples 
is like that of enamel in teeth, which appears to be precipitated 
upon the bone of the teeth under the guidance of a membranous 
case or mould. From a disordered fowl I have seen eggs produced, 
Vor. VI. N° JIL. M 
