36 



In support of the contention that Hydra is a modified 

 and not a primitive form, I would cite the following argu- 

 ments : — 



1. Hydra is hermaphrodite, being in this respect almost 

 unique among Hydrozoa. There is not the slightest 

 evidence for regarding a hermaphrodite condition as being 

 primitive among Hydrozoa, and there is very strong reason 

 for viewing it as secondary and acquired wherever it occurs 

 in other groups of animals. 



2. Hydra is fresh-water, differing in this respect from 

 almost all other Hydrozoa. Fresh-water forms are in most 

 cases derived from marine forms, and are very liable to 

 undergo modifications in consequence of their change of 

 habitat. 



3. The structure of the ovary of Hydra shows it to be in 

 a highly modified and not a primitive condition. Out of a 

 large number of primitive ova only a single one ripens, the 

 remainder serving merely to supply it with food. This is 

 an entirely exceptional and much modified condition, 



4. The other fresh-water genus, Cordylophora, is one in 

 which the shifting has already taken place to a very great 

 extent. It is a form which is believed to have only 

 recently become fresh-water, and it would not require a 

 very great amount of further modification to reduce it to 

 the condition of Hydra. 



5. The difference between the ovary of Hydra, which 

 involves ectoderm only, and the gonophore of an ordinary 

 Hydroid, which consists of both ectoderm and endoderm — 

 a difference which is fatal to a comparison of tlie ovary of 

 Hydra with a sporosac — becomes readily intelligible on the 

 above theory. 



