OF THE HYDROMEDUS^. 411 



Nutritive ( Nutritive Hydra ( Medusa-Bud <^ Eggs. 

 Hydra X < Blastostyle X < 



{ Defensive Hydra ( Medusa-Bud <^ Eggs. 



Nutritive I Nutritive Hydra ( Medusa-Bud <^ Eggs. 

 X. Hydbactinia Egg = Planula = Boot X ■ Hydra X •; Blastostyle X< 



( Defensive Hydra ( Medusa-Bud <^ Eggs. 



Nutritive t Nutritive Hydra ( Medusa-Bud <^ Eqgs. 

 Hydra X < Blastostyle * X - 



( Defensive Hydra, ( Medusa-Bud <^ Eggs. 



Now, what is the significance of this remarkable series of life-histories? Most of the 

 facts have long been known, but the most conflicting interpretations of them have been 

 advanced, and the student who seeks in the various monographs upon the subject an ex- 

 position of the relation between the direct development of a single adult from each egg - , 

 which is characteristic of most animals, and the circuitous history which is so remarkably 

 exhibited by the medusa', will find a speculative literature which is almost unlimited, but 

 a total lack of agreement as to the true solution of this, the most interesting of all the 

 problems involved in the life of these most interesting animals. 



The view which I believe to be the true one is that the remote ancestor of the hydro- 

 medusa' was a solitary swimming hydra, or actinula, with no medusa stage, but probably 

 with the power to multiply by budding. I believe that this pelagic animal gradually be- 

 came more and more highly organized and more perfectly adapted for a swimming life, 

 until it finally became converted into a medusa with a swimming bell and sense organs, 

 developing directly from the egg without alternation, but exhibiting during its growth 

 the stages through which it had passed during its evolution. After this stage of devel- 

 opment had been reached I believe that the larva derived some advantage from attach- 

 ment to other bodies, either as a parasite within other medusa', or as what may perhaps 

 be called a semi-parasite, upon other floating bodies such as the fronds of algae; and that 

 it multiplied asexually in this sessile condition, giving rise to other larvae like itself, all 

 of which became medusae. 



I believe that the sessile or attached mode of life of the larvae proved so advantageous 

 to the species, that it was perpetuated by natural selection, and that the primary larva 

 then gradually lost its tendency to become a medusa, but remained a sessile hydra, giv- 

 ing birth by budding to other larvae which became sexual medusae; and that the medusa- 

 characteristics of these secondary larvae were accelerated, and that the primary larva 

 gradually acquired, at the same time, the power to produce other larvae which remained 

 permanently, like itself, in the hydra-stage; that in this way the sessile hydra-communi- 

 ties with medUsa-buds and tree sexual medusae were evolved; and that finally these com- 

 munities became polymorphic by division of labor, and that the sessile habit proved .so 

 advantageous that the free medusae became degraded into medusa-buds, or sexual buds 

 on the bodies of the sessile hydras or on the blastostyles. 



The view which is most generally accepted is the reverse of this. Thus Huxley (34) 

 tells the student that the medusa is simply a reproductive organ which was originally ses- 

 sile upon the body of the hydroid, and that it has gradually acquired its free habit of life 



