24 Mr. W. K. Brooks on the Origin of 



of the formation of the umbrella is exactly parallelled by 

 innumerable similar phenomena in ,the lives of nearly all of 

 the higher Metazoa, and it therefore presents no difficulties ; 

 and if we imagine the gelatinous substance absent, the 

 raouthless, untentaculated, ciliated LiriopeAoxw^ is obviously 

 a planula with an outer layer of ectoderm and a central cap- 

 sule of endoderm. It has a spacious digestive cavity ; the 

 two layers are separated by a gelatinous substance ; and in 

 our species the cilia are restricted to a small part of the outer 

 surface ; but, in spite of these secondary modifications, it is 

 clearly a planula. It soon acquires a mouth and four solid 

 tentacles, and becomes converted into the floating hydra or 

 actinula, with ectoderm, endoderm, stomach, mouth, lasso- 

 cells, and four tentacles, but with neither subumbrella, sense- 

 organs, nor veil. This larva becomes converted into an adult 

 medusa by the growth of the tentacular zone into an umbrella, 

 and by the acquisition of sense-organs, precisely like the 

 -^giiiopsis-larva, and as each egg gives rise to only one 

 adult, the life history is simple and direct, with a planula- 

 f'tage, a hydra-stage, and a final medusa-stage, and it may 

 therefore be represented by the same diagram which was used 

 for JEginoiysis : — 



II. LiEiOPE : E(jci = Plumda = ActinuIa=Mi'dusa<^Eggs. 



In our common American Narcomedusa, Cunina octonarian 

 the fact that the larva is a true hydra was long ago pointed 

 out by McCrady. The planula-stage of this species has never 

 been observed; but the resemblance between the ciliated, 

 bitentaculated hydra and Metschnikoff's account of the JEgi- 

 wo^^szs-larva at the same stage is so close, that we have every 

 reason for believing that in this species also the hydra-stage 

 is preceded by a planula-stage without a mouth or tentacles. 

 The hydra soon acquires two more tentacles^ and is then fun- 

 damentally like the four-tentacled hjdra of Liriope. The 

 number of tentacles soon increases to eight, and the hydra 

 becomes converted into a medusa by the outgrowth of the 

 tentacular zone and the acquisition of sense-organs. So far 

 the life-history of our Cunina is as simple as that of ^gi- 

 nopsis or Liriope ; but it is complicated by the occurrence of 

 asexual multiplication in the larva and also by parasitism. 

 The actinula, or fioating ciliated hydra, after gaining access 

 to the subumbrella of a Turritopsis, gives rise to buds from 

 the aboral end of its body, behind the circlet of tentacles j 

 each of these buds is a hydra like the parent, and, like it, 

 becomes directly converted into a medusa. As these secon- 

 dar}^ h}'dras originate as buds, they are at first sessile ; but 



