480 
CCELENTERA TES. 
is evidence that this alternation of generations was, and indeed still is, widely 
spiead in both the animal and the vegetable kingdoms. It is however, as a rule 
STINGING CAPSULES. 
1 and 2, With retracted filament; 3, Partly protruded; 
4 Fully protruded. (Highly magnified.) 
suppressed, as animals rise in the scale of 
organisation. The Ctenophora are a highly 
developed group in which this alternation 
of generations has been suppressed. 
One word as to the changes neces¬ 
sary to turn the simple Cnidarian above- 
described into a bell or umbrella-like 
jelly-fish. The principal change is in the 
gelatinous layer between the outer skin- 
wall and the inner stomach or digesting 
layer. This middle layer develops into 
an enormous mass of glassy j^dy of such 
a shape that, instead of the body being 
long from the mouth (oral pole) to the 
bottom of the sac (aboral pole), the animal 
is umbrella-like, the mouth being under 
the bell, while the top of the bell corre¬ 
sponds with the old base by which the 
parent polyp was attached to the ground. 
Before taking the principal forms 
assumed by the Cnidaria, and briefly 
describing their relation to one another, 
the stinging - cells and batteries claim 
attention. These cells, though all micro¬ 
scopical, vary considerably in size, with¬ 
out their structure being essentially 
affected. The surface protoplasm of the 
cell is modified into a tolerably firm shell, 
enclosing an oval or cylindrical vesicle. 
Closely associated with this structure is 
a pointed process, standing up far above 
the level of the skin, known as the 
cnidocil. Within the vesicle is found, 
either spirally rolled or in an irregular 
tangle, a long filament, or hollow tube, 
which is a prolongation of the vesicle, but 
turned outside in. This tube, which is 
more than twenty times as long as the 
cell, is pointed at the tip, and almost up 
to the tip beset with two rows of fine, 
spirally-arranged, barbed hooks. When 
the cnidocil is touched or irritated, this 
filament is violently shot forth, being 
turned inside out like the finger of a glove. 
