NUTRITION AND EXCRETION 



99 



of Paramecium, in which the powers of movement, feeding, and 

 fission are affected and death ensues. Respiration and excretion 

 probably take place from the surface of the ectoderm and endo- 

 derm ; there is no special organ for either process. 



REPRODUCTION 



The species of Hydra reproduce themselves both sexually and 

 asexually. The green form is hermaphrodite, often protandrous, 







'^"K ■ V . A ■- "1 •»»>*! •■■ - 





^ afe 



• a 



X 





B 



Fig. 65.— Reproductive organs of the green hydra. 



In each case a testis is shown above, to the left, and an ovary below, to the right. In A the ovum is unripe, 

 in B it is ripe, has burst its covering of ectoderm cells, and hangs by a stalk. The large round spots 

 in the ovum are zoochlorellae. 



but the two British brown species have the sexes separate. 

 H. oligactis usually reproduces sexually in the autumn, the other 

 two in spring and summer. The generative organs are ectodermal 

 structures developed when sexual reproduction is about to take 

 place (Fig. 65). The ovaries, of which there is generally only 

 one in each individual, are found in the lower part of the body ; 

 the testes, of which there are several, are in the upper part. In 

 the early stages of both organs the interstitial cells multiply 

 and push out the musculo-epithehal cells so as to form a swelling. 



