CG@LENTERATA. 135 
The Hydrocoralline are peculiar in having a massive 
calcareous skeleton instead of the usual chitinous one and 
for their very primitive little medusoids. Their calcareous 
skeletons can be distinguished from the true corals by the 
absence of sef¢a in the apertures left by the polypes. The 
Siphonophora are floating pelagic colonies with little or no 
skeleton but with remarkable division of labour, the members 
of the colony being modified into a great variety of kinds. 
Crass II.—ScypuHozoa. (Actinia and Aurelia.) 
In these the ccelenteron at one time of their life is 
divided into central (gastric) and peripheral (vascular) 
cavities, and there is usually an ectodermal gullet. The 
gastric cavity usually has gastric filaments and the gonads 
are endodermal. Aurelia represents those types which have 
hydroid and medusoid phases, but a number of other jelly- 
fishes have only the medusoid phase. All these. form the 
sub-class Scyphomeduse. The important forms with only 
hydroid phase (¢g., Actinéa) form the sub-class Actinozoa. 
Actinia, like Hydra, is solitary and without an exoskeleton, 
but actinozoan colonies (like hydroid zoophytes) also occur. 
The skeleton, usually ectodermal, is most commonly of 
calcareous matter, and may assume vast proportions. 
These colonial types are called cova/s and their skeletons 
may be recognised by the presence of radial septa in the 
holes formerly inhabited by the polypes. (Coral Islands, 
see page 72.) The Actinozoa are divided into two important 
orders, the Hexactinia and Octactinia, according to the 
number of mesenteries and other structural features 
mentioned in the types Actnta and Alcyonium. 
Crass III.—CTENOPHORA. 
The unique motor organs of this class tend to separate 
them from the other two classes, but they are connected by 
certain intermediate forms. 
Cydippe is a very fair representative of the class. 
They are typically free-swimming pelagic organisms of 
carnivorous habits. Some (Cestum) become elongated in 
one plane to form a long ribbon, or they may (Bevoé) form 
a large bell by increase of the stomodzeum. 
