HYDRA AND OB ELI A. POLYPS AND MEDUSA 163 



animal to the surface it 

 hangs from. Each mus- 

 culo-epithelial cell has 

 a large oval nucleus. 

 In the tentacles these 

 cells are less tall than 

 elsewhere. Between the 

 narrow ends of the 

 musculo-epithelial cells 

 are spaces, which are filled 

 with small, rounded inter- 

 stitial cells. These form 

 a reserve from which, 

 in various circumstances, 

 any of the other cells of 

 the body can arise. Thus 

 they retain the undiffer- 

 entiated nature of the 

 germs and are sometimes 

 called indifferent cells. 

 Embedded in the con- 

 tinuous layer of the ecto- 

 derm are peculiar cells 

 known as cnidoblasts. 

 These are very numerous 

 in the tentacles, where 

 they lie in groups or 

 batteries (Figs. 102, 403), 

 but absent from the basal 

 disc. Each of them has 

 a pear-shaped body with 

 the narrow end at the 

 surface of the animal, 

 where there projects 

 from it a short process 

 known as the cnidocil. 

 On this side the cell 

 contains a pear-shaped 

 sac, called the nematocyst. 

 The narrow outer end of 

 the sac is tucked in and 



FIG. 102.— A diagrammatic, longitudinal 

 section of Hydra, magnified. — From 

 Shipley and MacBride. 



bat., Battery of nematocysts. Only a few of 

 these are shown ; they cover the tentacles ; 

 ect., ectoderm; end., endoderm ; ent. , en- 

 teron ;_/!, foot ; h.t., hollow of a tentacle \ov., 

 ovary ; st.l., structureless lamella ; t. testis. 



