112 CELENTERATA. 
On agitation of the water, Hydra contracts its body and 
tentacles till it becomes a round knob, but if left to itself it 
will soon expand again to its normal condition. 
Very often the body appears tu fork into two 
parts each of which has a ring of tentacles. 
One of these is a Jud which is destined later to drop off 
the parent. 
Interval 
Structures. 
Fig. 46.—TRANSVERSE SECTION OF Hypra (Magnified). (Ad zat.) 
Mesogloea. 
Ectoderm.— 
Endoderm ~ 
Ccelenteron. 
If the animal be killed and preserved and cut into trans- 
verse sections, a low-power examination of such sections 
reveals the fact that the whole body is a hollow sac, the 
internal cavity being known as the cw/enteron. The wall of 
the body is of two layers, the outer layer or ectoderm and the 
inner or endoderm, between which is a thin supporting 
lamella, the mesoglwa. 
The ccelenteron may occasionally contain the bodies of 
small animals which constitute the food of Hydra. 
On examination with a higher power of the microscope 
the endoderm cells prove to be arranged in 
asingle layer and the cells themselves are 
considerably larger than those of the ectoderm. Each cell 
contains a nucleus and a number of small bodies scattered 
through its protoplasm. 
Histology. 
