1893.] MICROSCOPICAL JOURNAL. 65 



Cyclops) comes in contact with a tentacle it is at once paraly/ed, 

 and the tentacle l)en(ls with it toward the centre of the maniil)rium, 

 whereupon the mouth opens and the prey is swallowed. Hydras 

 often swallow a prey lar<fer than their own bodv. so that the iuhe 

 is <;:reatly hulked out. After a certain period the shell}' remains 

 of the digested prey is pushed out through the mouth, one open- 

 ing thus serving as both mouth and anus. Some specimens of 

 Hydra, but not all, have one or more buds or smaller specimens, 

 more or less like themselves, attached to the sides of the tube, of 

 varying ages, from fully formed ones to the least perceptible swell- 

 ings. These subsequently break loose from the parent bodv and 

 grow to the paternal size and bud in their turn. Hvdra thus 

 grows, feeds, moves, feels, and reproduces. What animal can do 

 more ? 



If Hydra be killed, preserved, and prepared after microscopical 

 methoils and sectioni/.ed, its structure stands revealed. It is shown 

 in figures 2 and 3. both being somewhat diagrammatic ; fig. 3 less 

 so. 



It is composed of two distinct layers, an outer, ectoderyn, and 

 an inner, cndoderm^ bounding a central cavity, the stomach cavity. 

 These layers are composed of multitudes of similar units, each 

 one a celL being a bit of protoplasm with a nucleus, but no con- 

 tractile vacuole, and a thin outer wall, and hence comparable 

 with a protozoan. The cells of the ectoderm are smaller than 

 those of the endoderm. Thev cover the body in every part, in- 

 cluding the tentacle, manubrium, stopping short at the mouth, 

 and also run under the base, so that any part of the body which 

 could come in direct contact with the water has an ectodermal 

 covering. The deeper ends of the ectoderm cells are drawn out 

 into long motile processes which are contractile, and it is by the 

 aggregate actions of these in various directions that the motions 

 of the bod\ or tentacle are produced. The ectoderm in theoreti- 

 cal section is illustrated (fig. 15) after Von Lendenfeld.* 



This shows it to contain peculiar structures, called nettle cellsf 

 or nematocvsts, which contain a small sack filled with poisonous 

 fluid and furnished with a dart and a long hollow tube for con- 

 veying the poison into any bodv which may be pierced by the dart. 



The c\'st is surrounded by protoplasm whose contraction is ex- 

 cited h\ the sensitive hair at a greater or less distance, excited in 

 turn by contact with a foreign object. The ectoderm, besides 

 containing these muscular and nettle cells, is made up also of 

 simple columnar cells and of glandular cells, and its outer edge 

 presents a peculiar cuticular appearance, as if harder than the 

 rest, forming a sort of cuticle to protect the softer protoplasm 

 beneath (Fig. 5). The endoderm lines the body internally 

 throughout (cf. fig. 3), including the tentacles and the buds, 

 but it will be noticed that the gonads are thickenings of the ec- 



»2 Jl. Micros. Sci., Jan., 1887: :ilso Am. Niit., 1887, p. 289. 

 t E. B. Wilson. See this Journal, vol. ix, p. 79. 



