34-6 ELEMENTARY BIOLOGY. [CHAP. 



intestine; there are no organs of circulation, respiration or 

 urinary secretion; the products of digestion are doubtless 

 transmitted, by imbibition, from cell to cell, and those of 

 the waste of the cells exuded directly into the surrounding 

 water. While the Hydra has none of the special appara- 

 tuses which are termed glands, definite secretory cells are 

 nevertheless present. Among the more important of these 

 are those developed in the ectoderm of the foot which are 

 utilized for purposes of adhesion, and the secretory cells of 

 the endoderm, most numerous in the hypostome, which is 

 eversible. 



Nematocysts are generally, but not invariably, ejected, if 

 any portion of the body which bears them be touched. 

 Certain of the ectoderm cells, usually, if not always, lodging 

 nematocysts, bear each a stiff filament or cnidocil such as 

 can readily be seen in life projecting beyond the free surface 

 of the tentacles. Continuity has been traced between these 

 cells or cnidoblasts and certain small nerve-cells sparsely dif- 

 fused in the deeper layer of the ectoderm; the whole con- 

 stituting an elementary neuro-sensiferous apparatus, through 

 the agency of which control of at least the cnidoblasts and 

 their contained urticating capsules is exercised. 



The fully formed Hydra may further be compared to 

 those animals previously dealt with, at that stage in their 

 development when the body consists of a double-walled sac 

 (cf. especially the gastrula stage of the Snail and its repre- 

 sentative in the Crayfish). The inner layer gives origin, 

 in the latter, to the digestive epithelium and its appended 

 glands; in Hydra it forms the digestive layer. The ecto- 

 derm is, in all, protective, and from it such neuro-sensifer- 

 ous organs as are formed, exclusively arise. Between the 

 applied surfaces of the ectoderm and endoderm there is 

 interposed a gelatinous middle layer, which, under the 



