750 THE POPULAR SCIENCE MONTHLY. 



and the adult jelly-fish lives only long enough to effect the wide 

 distribution of the eggs, and the establishment of new colonies of 

 larvge. 



Something very similar to this has occurred in a few insects. 

 The caterpillar stage of most butterflies is simply a preparatory 

 step directed toward an end, the production of the perfect insect ; 

 but the bagworm is a butterfly in which the larval life is most 

 important, for, while the caterpillar lives long, the female insect 

 never escapes from her cocoon, but after her final transformation 

 lays her eggs within it and dies, while the male lives only long 

 enough to find and fertilize the female, and then dies also. 



In the case of the hydroids, the power of budding, a power 

 which is almost absent in insects, enables the larval life to assume 

 a degree of importance which it could not have if the larva re- 

 mained simple, for it has rendered division of labor possible, and 

 has produced polymorphic communities, most of the members of 

 which are out of the line of succession. The gradual reduction of 

 the adult life is also facilitated by the process of budding, as this 

 effects a great increase in the number of adults which come from 

 each Qgg, and thus secures the sexual reproduction of the species, 

 notwithstanding the shortening of the life of each adult. 



The shells of hermit-crabs often carry colonies of another hy- 

 droid, which is so similar to Dysmorphosa that a drawing of one 

 will answer for the other. They are almost exactly alike, and it is 

 only after careful examination that any difference between them 

 is discovered ; but, inconspicuous as the difference is, it is highly 

 important, for in the second form, Hydractinia, the adult locomotor 

 jelly-fish stage has been completely lost, and the whole life of the 

 species has become centered in the larvae. The blastostyles pro- 

 duce buds, which acquire some rudimentary traces of the organi- 

 zation of jelly-fish, but they never become free or complete their 

 development. While still on the blastostyles, they produce eggs 

 or spermatozoa, and having thus accomplished their purpose and 

 secured the perpetuation of their race, they die. The life of 

 Hydractinia is shown in Diagram VII : 



II Feeding hydra ( Medusa < eggs. 



' Feeding hydra x -] Blastostyle x < Medusa < eggs. 



( Defensive hydra ( Medusa < eggs. 



VII. Hydractinia. 



Egg = Planula = Root x 



i Feeding hydra ( Medusa < eggs. 

 Feeding hydra x •] Blastostyle x < Medusa < eggs. 



( Defensive hydra ( Medusa < eggs. 



I Feeding hydra I Medusa < eggs. 

 Feeding hydra x \ Blastostyle x -| Medusa < eggs. 



( Defensive hydra ( Medusa < eggs. 



This is by no means the end of the story, for the many species 

 of hydroids without any jelly-fish stage present all stages in the 

 gradual simplification of the sessile medusa buds, until at last all 

 traces of the structure of the jelly-fish disappear, and they are 



