HYDRA: ITS ANATOMY AND DEVELOPMENT. 243 
mass. It will remain attached to one spot for a long time, but it 
is capable of moving about, similar to that of the looping cater- 
pillar, and at other times floats passively in the water. 
Hydra is a tubular animal, and when found in water it is 
generally attached to a plant, such as duckweed, &c., or to other 
substances, as shown in fig 60. It is of simple structure, consisting 
of a body wall, which is composed of two membranes—an outer 
or ectoderm and an inner or endoderm—the outer wall being the 
protective and the inner the digestive organ. ‘The tentacles are 
tubular processes of the sac, and also are formed externally by the 
ectoderm, and internally by the endoderm. The body and arms 
have a warty appearance, which are formed by large cells, around 
which are a number of smaller ones. Upon close examination of 
the outer wall, there can be seen the following : Fig 61 (1c), large 
conical nucleated cells, with the broader end turned outwards. 
(s, c) Smaller rounded cells, packed between the deep ends of the 
large ones. (s, 0, c) Small oval capsules, with a filament coiled 
up inside them. (s, g) Small granules, consisting of a brown or 
black colour. Fig. 62 is a thin section showing the cells in the 
ectoderm and the filament uncoiled. (m,s) The thin muscular 
stratum between ectoderm and endoderm, sometimes called the 
Mesoderm. (c, c) The cells of the endoderm are large and nucle- 
ated, have a flattened edge, a rounded free end, are arranged in 
a single layer, and are ciliated. The Hydra possesses no sense 
organs or glands, and I may as well state for the benefit of the 
anti-vivisectionists that it can have no more feeling than a sensitive 
