V. 



The Lucernarians as Degenerate Scyphomedusae. 

 A Note upon the Phylogeny of the Order. 



PRELIMINARY. 



BEFORE entering upon discussion of the problem of the descent 

 of the Lucernariae, it may be well to set out for the guidance of 

 the general reader the more important facts already known concerning 

 the class to which they belong. 



The Hydrozoa, in which together with the Zoophytes, Jelly Fishes, 

 and Siphonophores they find place, is conveniently grouped in two 

 divisions (2). The first includes the Lucernarians themselves, together 

 with those fleshy Medusae or jelly fishes without velum (hence some- 

 times called " acraspedote medusae ") and with eight marginal sensory 

 organs (tentaculocysts) so well-known under the forms of Aurelia, 

 Pelagia and Rhizostoma (termed collectively Discomedusae), and finally 

 the Conomedusae, a small group with four tentaculocysts and with a 

 velum — Chavybdaa being the type. Such sub-class comprising these 

 orders constitutes the Scyphomedusae, so called on account of the general 

 presence of a particular stage in the life-history of typical species. 

 Normally this is as follows : — A ciliated embryo (proceeding from the 

 sexual or medusa stage) after a short period of free-swimming pelagic 

 existence, settles down and becomes attached to some stationary 

 object by one end. A mouth forms at the other, around which 

 sixteen finger-like tentacles appear ; and at the same time the lower 

 part of the body becomes drawn out into a stalk. 



This elegant form, T x ¥ to -J inch in height, is the scyphistoma stage, 

 and is what is sometimes termed the hydriform phase, from its 

 general resemblance to the fresh-water polyp, Hydra. Soon, as 

 growth goes on, food being captured by the transient tentacles, trans- 

 verse constrictions — stvobilation — cut the body into a rouleau of discs, 

 appearing thus as a tiny pile of sculptured plates. Each disc has 

 eight arms, bearing each a tiny tentaculocyst at the tip. This is the 

 strobila condition, and the discs composing it, as the constrictions 

 become deeper, are, one by one, broken off and sent out on a free- 

 moving life. Known at this period as ephyva, these organisms rapidly 

 take on the form of ordinary jelly-fishes, develop sexual organs, and 



