THE MORPHOLOGY OP THE SEXUAL ORGANS OF HYDRA. 327 



are in the great majority of cases derived from marine forms, and also 

 that they are very liable to undergo modification in consequence of 

 their change of habitat. 



3. The other fresh water Hydroid, Cordylophora affords very inte- 

 resting evidence. In the first place, there is very strong reason for 

 regarding it having only recently migrated from the sea and adopted 

 a fresh water habitat. 1 Then as regards its sexual organs Cordy- 

 lophora is in an extremely modified condition. The genital cells, 

 whether ova- or spermatozoa, are lodged when fully developed in gono- 

 phores which never reach even the disguised medusa condition, but 

 are arrested at a stage very little in advance of the sporosac. 2 



This, however, is not all, for Weismann has shown 3 that the genital 

 cells of Cordylophora do not arise in the gonophores, but in the body 

 walls of the primary zooids, where they may be recognised long before 

 the gonophores have commenced to develop, though on the appearance 

 of these latter they migrate into them. 



These facts seem to me to speak very strongly in support of the 

 view advanced above concerning Hydra. Cordylophora is a genus 

 which has only 'recently become a fresh water one, and in which the 

 condition of the reproductive organs is such that tvere the genital cells 

 to remain and ripen in the position in which they first appear ', i.e. in the 

 body wall of the zooid, they would agree exactly in all essential points 

 with those of Hydra. It is surely more reasonable to suppose that in 

 Hydra, whose fresh water habits have been longer established, a pecu- 

 liarity which we find already very highly developed in Cordylophora 

 should be carried just one step further, than that the two forms should 

 represent the opposite extremes of the series, for on Weismann's 

 view Hydra is the most primitive, Cordylophora the most modified of 

 Hydroids. 



4. Additional evidence of considerable value is afforded by the struc- 

 ture of the ovary itself in Hydra. In its early stages of development 4 

 this consists of a plate-like mass of interstitial cells which at first are 

 all of about equal size, but of which one only, situated in the centre of 



1 Concerning this migration, Semper states :— " It is, so far as I know, the only example 

 of an animal that can be proved to have originally lived in the sea or in brackish water, and 

 which, within our own time, has gradually accustomed itself to live in pure fresh water." 

 Animal Life, pp. 151-153. 



3 For good figures of these gonophores, vide F. E. Schulze, Veber den Bau und die Ent- 

 loicJclung von Cordylophora lacustris. PI. Ill & IV. 



3 Weismann, op. cit., 29-33. 



* For an account of the development of the ovum in Hydra, vide Kleinenberg, Hydra, 1872, 



