OF THE HYDROMEDUS.E. 419 



that the detached buds with generative organs should be locomotive, so as to distribute 

 the species as widely as possible, and such buds in connection with their free existence 

 would naturally acquire a higher organization than their attached trophosomes. It is 

 easy to see how, by a series of steps such as I have sketched out, a. division of labor 

 might take place, and it is obvious that the embryos produced by the highly organized 

 gonophores would give rise to a fixed form from which the fixed colony would be budded. 

 Thus an alternation of generations would be established as a necessary sequel to such 

 division of labor." He goes on to state his belief that the sessile medusa-buds are de- 

 graded medusa 1 , and that the medusae, which like Liriope develop directly from the egg, 

 are forms in which the hydra stage has disappeared from the developmental cycle; and 

 summing up his views he says that three types of development are presented by the Hy- 

 dromcdusae. 



1. No alternation of generations: permanent form a sexual hydra or hydra community. 

 Example: Hydra. 



2. Alternation of generations: hydroid stage fixed, medusa stage free. Example: 

 most hydroids. 



3. No alternation of generations: permanent form a sexual medusa. Example: 

 Trachomedusse. 



But, in his explanation, which we have quoted, he recognizes the following six succes- 

 sive stages in the evolution of the Hydromedusae. 



1. Solitary hydra, no polymorphism, all buds detached, all persons sexual. 



2. Community of sexual persons, giving rise also to detached buds which also become 

 sexual persons. 



3. Polymorphism, community of asexual nutritive persons, detaching buds which be- 

 come sexual persons. 



4. Same, detached sexual persons specialized as locomotor medusa 1 . 



5. Polymorphic community consisting of nutritive asexual persons and sexual ses- 

 sile medusae-buds, or 



6. Derived from 4, medusae without a hydra stage. 



Grobben (74) advocates a view which is very similar to that given by Balfour, and 

 he believes that the alternation has been produced by the following series of steps: 



1. Solitary hydroids, like Hydra. 



2. Communities without polymorphism, each individual sexual, like Ilydrella. 



3. Communities, with polymorphism, with sexual and asexual persons, all sessile. 



4. Same, with free reproductive persons. 



5. Same, with free reproductive persons specialized as locomotor medusa?. 



6. Same, with medusae degraded to sessile reproductive buds. 



7. Derived from 5, medusae without a hydra stage. 



"While Balfour believes that certain members of the community first became free and 

 afterwards became specialized for reproduction, Grobben believes that they became 

 specialized for reproduction while sessile, and that the tendency to become free was 

 afterwards acquired, but in other respects these two authors are in substantial agree- 

 ment. 



