15 



SUB-KINGDOM (TYPE} ILC(ELENTEEATA. 



EADIALLY symmetrical animals, in which the mouth opens 



into a simple or variously divided space, which repre- 



sents the alimentary tract, 



and which may or may not 



be divided into two por- 



tions, one specially con- 



nected with digestion, and 



the other corresponding 



with the body-cavity of 



the higher animals. Body- 



wall composed of two fun- 



damental layers (" ecto- 



derm " and " endoderm "). 



Nervous system sometimes 



specialised, sometimes dif- 



fused, but no vascular sys- 



tem developed. Eeproduc- 



tive organs invariably pre- 



sent at some period or 



another of life, though 



asexual reproduction is 



very general. 



CLASS I. HYDROZO A. 



Fig. 9. The Green Fresh- water Polype (Hydra 

 viridis), suspended head-downwards from apiece of 

 a stem of an aquatic plant, enlarged, a, One of the 

 tentacles ; 6, Testis or spermarium, with sperma- 

 tozoa in its interior ; c, A single large ovum, pro- 



SUB- CLASS I. - HYDKOIDA trading from the side of the body; d, Disc of at- 

 i-r-r j -j rr v \ tachinent (" hydrorhiza "). 



(Hydroid Zoophytes). 

 ORDER I. HYDRIDA. Hydra (Fresh-water Polype, fig. 9). 



(Kleinenberg, Hydra, eine anatomisch-entmckelungsgeschichtliche Unter- 

 suchung, 1872.) 



