THE CQSLENTERATES 33 
34. Hydrozoa, or Hydra-like animals.— Attention has al- 
ready been directed to the fact that the structure of Hydra 
is the simplest of the ccelenterates ; nevertheless, the thou- 
sand or more species belonging to this class which present 
a much more complicated appearance (Fig. 17) possess 
many fundamental Hydra-like characters. It is owing to 
this fact that this assemblage of forms has been placed in 
the class of the Hydrozoa, or Hydra-like animals. 
With but very few exceptions the members of this class 
are marine, usually living near the shore-line, where at 
times their plant-like bodies occur in the greatest profusion 
attached to rocks, seaweeds, or the bodies of other animals, 
particularly snails and crabs. Fig. 17 (upper colony) gives 
a good idea of one of the more complex forms, whose tree- 
like body attains in some cases the relatively giant height of 
from 15 to 25 c.m. (six to ten inches). In early life it bears 
a close resemblance to a Hydra. Buds form in much the 
same way, but they retain permanently their connection with 
the parent, and in turn bear other buds, until finally the form 
shown in the figure is attained. In the meantime root-like 
processes have been forming which afford firm attachment 
to the object upon which the body rests. Also during this 
process the cells of the outer layer form a horny external 
skeleton ensheathing the entire organism except the ter- 
minal portions (the hydranths, Fig. 18, B) bearing the ten- 
tacles. The gastric cavities of all communicate, and the 
food captured by one ministers in part to its own needs 
and, swept through the tubular stalks and roots, is also 
shared by all other members. 
35. Jelly-fishes and the part they play.—During the pro- 
cess of growth a number of stubby branches arise which 
differ from the ordinary type in shape, and also in many 
cases as regards color. These club-like, fleshy portions de- 
velop close-set buds (Fig. 18, c) which early assume a bell- 
like shape, the point of attachment corresponding to the 
handle, while the clapper is represented by a short, slender 
