48 R. T. Günther 



At what stage the Mncstra fixes i<^self to the Plnjllirlioè is still 

 a mystery, but a mystery whicli the facts come iiear to clearing up. 

 Amoug the very yoimgest individiials of PJ/ylHrhoe, a larg-er pro- 

 portion seemed to be devoid of the medusa than was the case 

 with the full grown individuals. The infereuce is that the Mnestra 

 medusa or larva attaches itself to the PliijlUrìio'é after leading a 

 free swimmiog existence of longer or shorter duratiou. 



If this hypothesis be accepted, it follows as a necessary conse- 

 quence that the numbers of young Mnestra must far exceed the 

 numbers of PJu/lUrJwe, since but a small proportion can ever succeed 

 in settling upon a Phyllirhoe in the open ocean. It also follows 

 that the powers of reproduction of the Mnestra must be immensely 

 increased as compared with the generality of medusae, and must, 

 one would think, be greater than is usuai even among raany para- 

 sites. It is therefore most significant that ali who bave hitherto 

 examined Mnestra should bave failed to discover any trace either 

 of sexual or asexual reproduction. 



To repeat: 



1. If our inference be correct that Mnestra passes through a 

 free swimming stage of some duration, it follows that free 

 swimmiug individuals should be very abundant. 



2. Its method of reproduction should be much in evidence. 



But, neither bave free swimming Mnestra medusae been captured 

 in considerable numbers, nor has the mode of reproduction of the 

 sessile individuals been recorded. 



Such considerations together with a knowledge of rei)roductive 

 l)roce8ses characteristic of parasites led me to the conclusion that 

 the reproductive process of Mnestra might occur, not in the open 

 sea but within the body of the Pki/llirhoe. The migratory pro- 

 pensities of the germ cells of the Hydrozoa are univcrsally acknow- 

 ledged: is there any difficulty in imagining that germ cells which 

 bave originatcd in the manubrium of Mnestra are attractcd by 

 pleutiful nutrimcnt and wander into the nutrient liquids within the 

 PhijUirhoe? 



Ötimulated l)y this speculation I immediately rccxamined ali 

 available s]»e(*iniens of PJft/llirJfoe, and recognised in many, struc- 

 tiires which bear the greatest resemblauce to ova, 8})eimatozoa and 

 segmenting and dividing embryos (figa. 25 — 35). 



