12 GENERAL SURVEY OF THE ANIMAL KINGDOM. 
‘ectoderm or epiblast, internal endoderm or hypoblast lining the gut, 
and a median mesoderm or mesoblast lining the body cavity). In the 
next two classes (Ccelentera and Sponges) the conditions are different, 
-as may be expressed in the following table :— 
Sponces AND CG&LENTERA. HIGHER ANIMALS (C@&LOMATA). 
There is no body cavity. There is but | There is a body cavity or cwlom be- 
one cavity, that of the food canal. tween the food canal and the body- 
wall. But this is often incipient, or 
degenerate. 
Except in ctenophores, there is no | There is a distinct middle layer of cells 
definite middle layer of cells (meso- (mesoderm) between the external 
derm), but rather a middle jelly ectoderm and the internal endo- 
(mesogloea), and the embryo is derm. The embryo is triploblastic. 
diploblastic. 
The radial symmetry of the gastrula | The adults are usually bilateral, in some 
embyro is usually retained in the cases asymmetrical, in echinoderms 
adult, and the ‘longitudinal (oral- superficially radial. 
aboral) axis of the adult corresponds 
to the long axis of the gastrula. 
Coelentera.—This series includes jelly-fishes, sea-anemones, 
corals, zoophytes, and the like, most of which are equipped 
FIG. 16.—Sea-anemones on back of hermit-crab, 
—After Andres. 
with stinging cells, by means of which they paralyse their 
prey. All but a few are marine. The body may be a 
tubular polyp, or a more or less bell-like “ medusoid,” and 
