40 ANIMAL LIFE 
of these cells. The ectoderm cells do not take food 
directly, but receive nourishment only through the endo- 
derm cells. 
Hydra is not permanently attached. It holds firmly 
to the submerged stone or weed by means of the sticky 
secretion from the ectodermal gland cells of its base, but it 
can loosen itself, and by a slow creeping or gliding move 
along the surface of the stone to another spot. Even when 
attached, the form of the body changes; it extends itself 
longitudinally, or it contracts into a compact globular mass. 
The tentacles move about in the water, and are continually 
contracting or extending. 
Like Volvoz and the sponges, those other slightly com- 
plex animals we have already considered, Hydra has two 
methods of multiplication. In the simpler way, there 
appears on the outer surface of the body a little bud which 
is composed, at first, of ectoderm cells alone; but soon it is 
evident that it is a budding, or outpushing, of the whole 
body wall, ectoderm, endoderm, and middle membrane. In 
a few hours the bud has six or eight tiny, blunt tentacles, 
a mouth opening appears at the free end, and the little 
Hydra breaks off from the parent body and leads an inde- 
pendent existence. in the more complex way, two kinds of 
special reproductive cells are produced by each individual, 
viz., large, inactive, spherical egg cells, and small, active 
sperm cells, each with an oval part or head (consisting of 
the nucleus) and a slender, tapering tail-like part (consist- 
ing of the cytoplasm). The egg cell lies inclosed in a layer 
of thin, surrounding cells, which compose a capsule for it. 
When the egg cell is ready for fertilization this capsule 
breaks, and one of the active sperm cells finds its way to 
and fuses with the egg cell. The fertilized egg cell now 
divides into several cells, which remain together. The 
outer ones form a hard capsule, and thus protected the 
embryo falls to the bottom, and after lying dormant for 
awhile develops into a Hydra. 
