70 
POPULAR SCIENCE REVIEW. 
of the sarcosome which immediately overlies the retracted indi- 
vidual_, we perceive that it is marked by eight horizontal groves 
or depressions which radiate from the centre^ in which is ob- 
served a very slight aperture (PL Y. fig. 2). The latter it is which 
widens when the polypite extends himself, and it then presents 
a large orifice beautifully crenated at its borders (Fig. 3 cl), the 
eight groves having as it were been unfolded so as to form as 
many elegant festoons which correspond in position to the 
creature^s tentacles^ which are of exactly the same number. 
The object of this arrangement is obvious; it allows the ten- 
tacles alone to be protruded when the creature is disinclined to 
extend himself to any distance from the outer surface of the 
sarcosome. Turning now to the polypite itself^ we find it to 
consist of a tube (c) of an exceedingly delicate transparent mem- 
brane^ the upper part of which expands a little so as to form 
a kind of disk (6)^ which is termed ih.Q ]jeristome {peri, around; 
stoma, the mouth) ; to this structure the tentacles are attached, 
and form an elegant and radiating crown which surrounds the 
mouth. These tentacles are not tubular and simple as in the 
sea-anemone, they are somewhat fiattened horizontally, and 
have two rows of smaller plumules projecting from their 
borders, these being in their turn faintly tuberculated. The 
plumules do project rectangularly from the edges of the ten- 
tacles, but are arranged in a somewhat oblique manner, and 
are directed in a line from above downwards and from within 
outwards. They can be moved separately or collectively at 
the will of the animal, and are sometimes arranged round the 
mouth like the spokes of a wheel, while at others they form a 
graceful, vase-like group. When a thin vertical section of a 
tentacle is placed under the microscope, the arrangement of 
the tissues can be very clearly observed. There are two dis- 
tinct layers of membrane, an outer and an inner : the former 
is made up of a fine granular basis embedding small oval cells 
and having scattered among them larger vesicles which ap- 
pear to have striated walls ; these especially are important, for 
they are the bodies known as thread-cells, in fact, the stinging 
organs which are found so abundantly and well developed in 
the jelly-fishes and which procured for that group the name of 
Acaleplicje or sea-nettles. When one of these is isolated it is 
seen to be in reality a double cell (an external or protective 
envelope, and an internal case in which the peculiar stinging 
thread is coiled) whose walls are perfectly concentric. The 
inner membrane is composed of cells of a very large size, 
and which are not packed closely together as in the 
former case, but are arranged in rows which form curious 
polyhedral spaces ; these cells are clothed with cilia (delicate 
filaments), and undoubtedly share in promoting the movement 
