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 Volvox 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. 



