142 



BIOLOGY 



B 



FIG. 69. HYDRA 



A, an animal in its expanded form; B, the same animal contracted; C, a diagram of the 

 longitudinal section of the animal, showing the internal structure; D, an epithelio-muscle 

 cell; E, a bit of the body wall highly magnified showing the two layers of the body; F, a 

 digestive cell; G, one of the nematocysts with its thread extruded; H, a second type of 

 nematocyst with the coiled thread within the sac; 7, nematocyst of the third type with its 

 thread extruded; J, a bit of the tentacle, very highly magnified, showing the batteries of the 

 nematocysts; K, two of the secreting cells of the basal disk. 



en, cnidocil; 

 ec, ectoderm; 

 n, endoderm; 



m, mouth; 

 mes, mesoglcea; 



o, ovary; 

 s, spermary. 



