THE GROWTH OF JELLY-FISHES. 



747 



Hydra x 



V. TuRRiTOPSis. — Egg = Planula = Root x \ Hydra x 



I 



[^ Hydra x 



\ Medusa < eggs. 

 { Medusa < eggs, 

 j Medusa < eggs. 

 [ Medusa < eggs. 



Medusa < eggs. 



Medusa < eggs. 



] 



The life-history of Turritopsis is therefore like this, and the 

 chain which connects the egg with the adult is broken three 

 times, for the root, which is directly derived from the egg, goes no 

 further, nor do the hydras which bud from the root become jelly- 

 fish, and the latter form still a third set of individuals. 



The larval life is long and important ; the number of sexual 

 adults produced by each egg is very great indeed, and the life-his- 

 tory is extremely complicated, but each one of the individuals is 

 in the direct line of succession ; for, while neither the root nor the 

 hydras ever become converted into any higher form, the root pro- 

 duces hydras, and each one of these produces jelly-fish. 



In the next species to be considered, a Eutima which is com- 

 m.on on our coast (Fig. 16), another stage of complexity is intro- 

 duced by the restriction of the power to bud jelly-fish to certain 

 hydras, while others become specialized for nutrition. This spe- 

 cialization has come about gradually, and the various species of 

 living hydroids exhibit all the steps in the process. In some spe- 

 cies, as in Turritopsis, all the hydras perform both functions, and 

 are alike in structure ; in oth- 

 ers, those which are placed at 

 the tips of the branches and 

 are best able to obtain food 

 devote themselves to this pur- 

 pose and produce no jelly- 

 fish, while these are budded 

 only from those hydras which 

 are near the base of the col- 

 ony. In some cases the two 

 sets of hydras are alike in 

 structure, but in other species 

 the feeding hydras at the tips 

 of the branches are very large, 

 with capacious stomachs and 

 long tentacles, while the re- 

 productive hydras have small 

 tentacles and mouths. In still 

 other species, as in Eutima, they are true blastostyles, without 

 mouths, and with rudimentary tentacles, and all the work of nu- 

 trition is performed by the feeding hydras. 



The planula of Eutima is shown in Fig. 17. After a short swim- 

 ming life, it fastens itself to some solid body, and elongating, be- 



FiG. 17. 



