420 W. K. BROOKS ON THE LIFE-HISTORY 



Hamann (32), however, adopts a somewhat different view and says that it was possi- 

 ble to believe that the alternation, or the origin of the medusa on the hydroid, came 

 about through division of labor, so long as it was supposed that the reproductive elements 

 originate in the medusae; but the discovery that the eggs, in many cases, originate in 

 the coenosarc of the hydroid and migrate into the medusa-buds, shows, he says, that this 

 is not the true view, and he advances another which was suggested to him in conversa- 

 tion with AVeismann. Starting with a community in which the reproductive elements 

 may originate in every part, he supposes that certain persons were set free from the 

 stock as in Hydra or in Tiarella, and that the persons thus set free were at first driven 

 about by wind and tide, obtaining their food by the use of their tentacles; that they were 

 simply floating hydras. Those which became adapted to this new life would, retaining 

 their power to produce eggs, give rise to fixed communities, in which locomotor persons 

 would be set free earlier and earlier, until finally the reproductive function would he- 

 come restricted to the free stage which would gradually acquire a locomotor apparatus 

 and thus become a medusa. 



These various opinions which are selected from a great number which might be quoted 

 show that the K polymorphism" hypothesis, which is the one most generally accepted, is 

 itself polymorphic and that authorities are far from an agreement as to the precise form 

 which it should take and this lack of agreement is in itself sufficient to excite a suspi- 

 cion that it may be merely an hypothesis unsupported by proof. 



All these authors agree, however, in the opinion that the reason for the evolution of 

 the locomotor medusa is the advantage which comes from the distribution of the sexual 

 elements and embryos, and the analogy of the polymorphic hydroids seems at first sight 

 to be a reason for believing that the medusa has originated according to the law of divi- 

 sion of labor. The hydroid blastostyle is undoubtedly a hydranth which has in this way 

 lost its nutritive function, and has become exclusively a reproductive zooid, while the or- 

 dinary hydranths have lost their reproductive function and have become simply nutritive 

 persons, and there is every reason for believing that the polymorphism of such a hydroid 

 as Hydractinia has been brought about by division of labor; but is there any real anal- 

 ogy between a blastostyle and a medusa? The medusa is very far from being, like the 

 blastostyle, a reproductive zooid. The blastostyle has no mouth, but the medusa is a 

 highly voracious animal, furnished with organs for perceiving and capturing its prey, 

 and with highly developed digestive organs. There is nothing in the structure of a 

 medusa to indicate that it is a reproductive zooid. It is true that in a few Tubularians 

 such as the Eucopella, recently described by Lendenfeld, or in Corymorpha, it is simpli- 

 fied in structure and is little more than a locomotor reproductive pouch; but these cases 

 are plainly the result of recent modification, and the typical medusa has all the character- 

 istics of a perfect adult animal with all the powers necessary for a complete life, and in 

 many species it produces other medusae by budding. There is certainly nothing in its 

 own structure to indicate that it has like a blastostyle originated by division of labor. 

 It does not show any tendency to lose its nutritive function, and its locomotor and 

 sensory functions are not lost, as Ave should expect them to be in a zooid specialized for 

 reproduction, but they are, on the contrary, much more highly developed than they are 



