THE ANIMAL KINGDOM 



393 



swinging the latter to a new position for attachment. Letting 

 go the hold by the tentacles, the hydra rises to an upright posi- 

 tion. In strong contrast with sponges, the hydra is capable of 

 quick energetic movements of its body and tentacles. As 

 already noted, it may in some cases detach itself and float about 

 by means of a mucus-enclosed gas bubble formed at its base. 



Reproduction in Hydra, referred to in an earlier connection 

 (p. 155), may be asexual (budding or fission), or sexual. The 

 germ cells of Hydra and the lower coelenterates are said to arise 

 from interstitial cells of the ectoderm. In the higher coelen- 



Fig. 222. — Obelia. g, gonosomes and gonotheca; h, hydranth and hydrotheca. 



terates they come from similar cells in the endoderm. In the 

 face of these facts, it is difficult to hold that a line of germinal 

 continuity or a germ track of cells as distinguished from the 

 somatic cells, exists in these animals, unless one assumes that the 

 interstitial cells are undifferentiated cells that have retained 

 the reproductive potencies of the egg. In Hydra the testis 

 develops as a swelling in the ectoderm of the body wall just below 

 the tentacles; the ovary develops as a similar swelling near the 

 base (Fig. 221). A single testis develops numerous spermatozoa; 

 a single ovary produces a single egg. The egg is fertilized in the 

 ovary and undergoes development as far as gastrulation within 

 the ovarian epithelium, after which it escapes and grows into an 



