336 



ANIMAL BIOLOGY. 



[Part II. 



after a short struggle, its activity ceases. The tentacle is 

 meanwhile retracted (Fig. 99, C.) ; other tentacles bend round to 

 lend assistance ; and the animalcule is tucked into the mouth, 

 and thus passes into the body-cavity, which occupies the whole 



\.s. 



FIG. 99. HYDRA VIRIDIS. 



A. Hydra half retracted, with a bud and an ovum attached to the 

 shrunken ovary. 



B. A small hydra firmly retracted. C. A hydra fully extended. 

 6. Bud. /. Foot. h. s. Hypostome. ovm. Ovum. ovy. Ovary, t. Ten- 

 tacles, ts. Testis. 



of the interior of the polyp. Here ere long it breaks up ; the 

 nutritive matter is absorbed by the lining cells of the cavity, 

 and the indigestible residue is got rid of through the mouth, 

 for there is no separate vent. 



Histology. In Fig. 100, A., a diagrammatic longitudinal sec- 

 tion through the body-wall of hydra is given. It is divided into 

 two main layers, ectoderm (ec.) external, and endoderm (en.) 

 internal. In the ectoderm there are large conical neuro- 

 muscular cells (n. m.). If a hydra be teased (fresh, or after 

 treatment with 20 per cent, acetic acid, or 1 per cent, osmic, or 

 dilute chromic and osmic) some of these cells at C. will be 

 readily seen. At their pointed ends they give off one or two 



