Metagenesis among the Hydromedusoi. 23 



ally lost the hydra-stage, so that they now develop directly 

 from the ^^g. I show that there is no reason for this opinion, 

 but that we have in Liriope among the Trachomedusfe and in 

 JEgineta and Cunina octonaria among the Narcomedusge a 

 true planula-stage and a true hydra-stage, although the hydra 

 is simply a larva which develops into a medusa by direct growth 

 and metamorphosis without alternation of generations. The 

 life-history of these forms proves conclusively that the medusa- 

 stage is older than the sessile hydroid cormus, which has 

 arisen through the power to multiply asexually which is 

 possessed by the hydroid larva of the medusa. 



We have among the existing hjdroids the series of stages 

 in the origin of metagenesis which are represented in the 

 following diagrams, in which the sign = denotes direct 

 metamorphosis without multiplication, the sign x denotes 

 asexual multiplication, and the sign < denotes sexual multi- 

 plication. 



In JEginopsis, as Metschnikoff shows, the q,'£^<^ gives rise 

 to a ciliated swimming planula, which acquires a mouth and 

 tentacles, and thus becomes directly and gradually converted 

 into a floating hydra or actinida, which is at tirst ciliated like 

 the planula. The tentacular zone of the floating hydra now 

 grows out into a flange or umbrella, which carries the tentacles 

 with itj sense-organs and a veil are soon acquired, and the 

 hydra becomes a medusa. 



The whole process is perfectly simple and direct ; there is 

 nothing like an alternation of generations, and the single Qg^ 

 becomes a single medusa with an actinula-stage, a floating, 

 hydra-like, larval stage, and a swimming medusa-stage. The 

 life-history is as simple and uninterrupted as that of any other 

 animal which undergoes a metamorphosis, and it may be 

 represented by the following simple diagram : — 



I. .Eginopsis : Egg=.Plunula=^Actinula = Medusa<::Eg(/s, 



As the floating hydra-stage of Tuhularia is well known 

 under the familiar name Actinula, and as it seems desirable 

 to use a special term for the free hydra-stage of Medusa; as 

 distinguished from a sessile hydroid, I shall employ this word 

 for this purpose, designating by it a free or floating hydra, 

 which may or may not be ciliated. 



I have shown that we have in Liriope and its allies a life- 

 history which is very similar to that of yEginopsis^ with 

 numerous secondary modifications, most of which are due to 

 the fact that the gelatinous substance of the umbrella begins 

 to be secreted between the endoderm and the ectoderm at a 

 very early stage in the life of the embryo. The acceleration 



