32 
POPULAR SCIENCE REVIEW. 
modelled upon these. During- inhabitation the upper margin of 
the case is turned inwards ; and when the animal suddenly and 
strongly contracts itself, the top of the case is somewhat drawn 
in after it. But this is not the result, as has been stated, <jf any 
adhesion of the margin to the animal, but simply that of the 
action of the water rushing into the vacuum suddenly produced 
by the downward retraction of the body, and carrying in with 
it the soft and flexible margin of the case. 
The substance of which the case is composed is so delicately 
transparent that it is with some difficulty made apparent to the 
eye; indeed, only by its fine filmy outline. In old specimens, how- 
ever, it acquires a brownish tinge, and much extraneous matter 
adheres to it. It is flexible, but not at all elastic; and appa- 
rently tough, without being viscid. Though, doubtless, a pro- 
tection to the animal, yet, as with higher creatures, this 
advantage is not without its dangers. I once saw a Stephano- 
ceros whose case had been accidentally bent down quite to a 
right angle, with many folds at the bend. The animal had no 
power to straighten it, but protruded, with its foot correspond- 
ingly bent. I have seen another, which, by some misadventure, 
had got one of its arms entangled in the substance of the mar- 
gin of the case. It could not by all its efforts free the arm ; 
and, after much apparent annoyance, during which this member 
evidently became diseased, the animal spontaneously forsook 
the case. Thus it remained, the foot (wrinkled up, indeed) 
projecting at a tangent, and the arm still fastened to the edge 
of the case by its ciliary setae. Under these unnatural circum- 
stances it soon died. While I am on these accidents, I may 
mention the example of another individual which voluntarily 
withdrew from its case. It Avas an operation of considerable 
labour and time ; when the animal was clear of its tenement it 
sloAvly moved along by alternate elongations and contractions, 
but had no power of swimming. The foot presently wrinkled 
up into a distorted form, and the animal survived only a few 
hours. Doubtless, these forsakings are the expressions of the 
creature’s uneasiness and an indication of disease. Such acci- 
dents should always be eagerly watched by the naturalist, as 
he frequently obtains an insight into structures and functions 
when these are modified by accident or disorder, such as no 
amount of patient investigation would have afforded him in the 
normal condition of his subject. 
The summit of the case commonly reaches as far as the neck, 
or to the base of the arms of the animal when fully extended ; 
and it is attached by its base, around the foot, to the support, 
as the leaves of Mijriopliyllum in my specimens. Thus the 
animal is completely enclosed to the height just named, and the 
only exit for the contents both of the ovary and the digestive 
