148 



THE RISE OF ANIMAL LIFE 



■-•<- \. 



S 



Fig. 8-12. Only the posterior portion is shown of this 

 hydra bearing several ovaries. Several eggs have al- 

 ready matured and have dropped off leaving only 

 the "cups" where they were attached. A newly formed 

 ovary, evident at the anterior end, is in the process 

 of producing an egg. This is a stained specimen. 



although as a rule not 



on the same animal 



at the same time. In formation of the sperm 



interstitial cells lying in the ectoderm 



undergo a series of divisions, forming a 

 protuberance which gradually grows large 

 as the sperms mature. A sexually mature 

 hydra may have several "ripe" testes along 

 its walls which resemble miniature mam- 

 mary glands, "nipples" and all (Fig. 8-11). 

 An opening appears in the "nipple" end of 

 the gland and the mature sperm cells swim 

 out into the surrounding water in search of 

 a mature egg found on another individual. 

 The sperms live and remain active for a 

 day or two. 



The eggs develop from interstitial cells 

 also, the difference being that only one 

 egg is formed in an ovary, whereas thou- 

 sands of sperm cells are produced in a 

 single testis. Several eggs usually form at 

 the same time on one hydra, giving it an 

 unusual appearance (Fig. 8-12). The eggs 

 at first resemble large amoeboid bodies. As 

 they mature they become spherical in 

 shape, resting on the outer wall of the 

 hydra attached to a cup-like depression in 

 the epidermis. 



After fertilization, cleavage is immediate 

 and continues until a hollow blastula forms. 

 An outer resistant shell then develops and 

 simultaneously the cavity (blastocoel) is 

 filled with cells from the lining. The young 

 embryo then drops off and lies quietly until 

 favorable conditions eventually arise, when 

 it emerges as a very small hydra with blunt 

 tentacles. Sexual reproduction usually oc- 

 curs in the fall of the year and seems to be 

 a safeguard for passing the winter months 

 because the young embryo resides in a cap- 

 sule which resists adverse temperatures. 

 Temperature is apparently the controlling 

 factor because sexual organs can be in- 

 duced in hydra at any time by reducing the 

 temperature. 



Hydra possesses no medusa stage and 

 hence does not exhibit metagenesis, which 

 is common among most Hydrozoa. The 

 next representative is a more typical ex- 

 ample of this class and is discussed pri- 

 marily for that reason. 



