114 OUTLINES OF EVOLUTIONAKY BIOLOGY 



female animal possessing either male or female gonads, while a 

 hermaphrodite animal may either possess a combined ovo-testis 

 or both ovaries and testes separately. 



In illustration of these points we may briefly describe the 

 structure and life history of the common fresh water polype, 



FIG. 57. The fresh water Polype (Hydra) cut in half longitudinally and 

 greatly enlarged. (From Marshall and Hurst's " Practical Zoology.") 



A, mouth ; B, hypostome ; C, digestive cavity ; D, ectoderm ; E, mesoglcea ; F, endoderm ; 

 G, tentacle ; H, testis ; I, ovum in ovary ; K, bud ; L, foot. 



Hydra (Fig. 57), so frequently found attached to aquatic plants 

 in ponds and ditches. Hydra is a member of the great group 

 Coelenterata, which includes the sea-firs, jelly-fish, sea-anemones 

 and corals, and which are distinguished by the fact that they 

 retain throughout life the fundamental features of the gastrula 

 (compare Fig. 13, IX, X). The body of a typical Ccelenterate 

 animal consists essentially of a simple sac (Fig. 57) whose wall 



