264 



GENERAL ZOOLOGY 



The number of tentacles a specimen of Hydra viridis may 

 have depends upon its size, and also upon its age. The small 



individuals are gener- 

 ally the youngest; 

 these have four tenta- 

 cles. The larger, older 

 ones gain in the num- 

 ber of tentacles, up to 

 the extreme number of 

 eleven; but there is a 

 greater per cent of in- 

 dividuals with six ten- 

 tacles than with any 

 other number. 



Internally, Hydra is 

 the simplest animal 

 we have studied. The 

 mouth (Fig. 131, 1) 

 opens into a cylindrical 

 cavity (Fig. 131, 7) with 

 no indication of gullet, 

 or radial partitions. 



FIG. 131. Fresh- Water Polyp. Longitudinal The cavity extends by 



slender tubes out into 

 each tentacle, and in all 



,gastrovascular cavity ; 8, spermatozoa form- parts Only two layers 



of cells separate it from 

 the water outside. The 

 inner one of these two layers is the endoderm (Fig. 131, 6), 

 and the outer one, the ectoderm (Fig. 131, 5). Cells of the 

 endoderm secrete fluid for digesting food, and even take up 

 small particles of food, digesting them inside the cell-sub- 

 stance. The ectodermal cells are modified into nettle-cells, 

 nerve-cells, muscle-cells, and general surface-cells. 



9 



section. Much enlarged 



1, mouth; 2, tentacle; 3, forming bud; 4, older 

 bud ; 5, ectodermal cells ; 6, endodermal cells ; 



ing ; 9, egg forming 



(From Parker's Elementary Rioloyy} 



