54 
as, e.g., in the claw through an aperture many times less than 
the member itself. And lastly, at all events in the higher Crustacea, 
impregnation only takes place when the female is in the soft state. 
The observations which follow, being taken from so few specimens, 
must not be taken as conclusive, but merely as preliminary to 
general research. 
The Shell. — The shell itself is composed of four layers, the 
intimate structure of which we shall not enter into at present. 
On the surface is a thin transparent cuticle. Beneath this is a 
somewhat broader layer which is deeply pigmented and gives the 
shell its characteristic colour. This layer can be easily scraped 
off the surface with a knife. The third layer is calcareous, and 
constitutes almost the total thickness of the shell. The fourth 
layer is again thin, and, like all the others, is made up of 
parallel lamellae. Beneath this, we find a regular layer of long 
cells which are much enlarged at the casting period — the ectoderm 
or chitinogenous layer of various authors. Beneath this layer again, 
we find the connective tissue, with glands, muscular fibres, vessels, 
and nerves. The superficial layer of the connective tissue is 
pigmented and constitutes the dermis. At casting, the separation 
takes place between the fourth layer and the ectoderm. A gelatinous 
mass, with no definite structure, is secreted in the intervening space. 
The cells of the ectoderm immediately scale off at their upper 
surface forming the upper layer of the new shell. The new 
shell is thus seen to be forming before the old one is thrown 
off. The cells of the dermis and ectoderm are much swollen at 
this period as is evidenced by the fleshy nature of the soft 
envelope of the crab shortly after casting. On treatment with 
alcoholic tincture of iodine, these cells are seen to be stored with 
glycogen (Cuenot il ). After the new shell is produced, these cells 
degenerate till, in the hard condition, the ectodermal epithelium 
can scarcely be distinguished. The muscle fibres which serve for 
attachment pass right through the dermis and are attached to 
the basal membrane formed of the interlacing fibres of the lower 
ends of the epithelial cells. In this way we see that the muscular 
attachment is never really severed when the exoskeleton is thrown 
off. 
The Blood. — On making an incision through the skin of a soft 
crab, an immense quantity of albuminous fluid can be withdrawn by 
2 . - Williamson : Contributions to the I af«* History of the Fdible ('rub. Kiyditeentli 
Annual Report of the Fishery Board for Scotland. Bart III. 1900. 
