EVOLUTION OF THE LUCEENARIANS. 7 



the Discomedusae, i.e. prior to their adoption of the two well marked 

 stages hydriform and medusiform that now characterise their life- 

 history. Quite recently, however, Dr. C. Herbt. Hurst and the writer 

 arrived independently (" Natural Science" Sept. 1893) at the novel 

 conclusion that the connection between the Lucernarians and the 

 Discomedusaa is much more recent and close. Thus we believe from 

 the review of considerable evidence that the former are descended 

 from a form having as well marked medusa-stage as Aurelia has, and 

 with well developed marginal sensory organs. Then by abbreviation 

 in the life-cycle, by a certain hastening of events, there were developed 

 genital products in the hydriform or scyphistoma fixed form. This 

 did away with the necessity for a free swimming sexual stage, and 

 being found advantageous, was permanently adopted by some the 

 immediate predecessors of the Lucernarians of to-day. 



Marginal sensory organs (e.g. tentaculocysts) can be useful only 

 among swimming forms (Hurst " Nat. Sc" vol. ii, part 16). Under 

 the changed conditions they became useless in the Lucernarians and 

 tended to become " vestigial." Such functionless organs are very 

 subject to variation and the writer has found such to a really 

 extensive degree in these bodies in Haliclystus. It is well known 

 that the origin of tentaculocysts is from ordinary marginal tentacles, 

 and it is to this form that the marginal bodies (c. cystophores) of the 

 species under description usually reverts. All gradations from the 

 normal form of c.-c. up to the normal tentacle can be traced. The 

 capitate terminations of the tentacles contain great numbers of 

 nematocysts or stinging cells useful in paralizing prey, arid the first 

 stage in reversion is marked by the appearance on the apex of a 

 c.-c., of a tiny wart containing numerous nematocysts (Fig. 4). The 

 next shows a larger wart. In Fig. 5 this has a tiny stalk, while in 

 Fig. 6 there is a well pronounced peduncle. Again in Fig. 7, is 

 drawn a tentacle one from a bunch of normal ones where the 

 stem is bellied out, in fashion approaching the most abnormal of 

 the colleto-cystophores. 



EXPLANATION OF FIGS, 1 TO 9, PLATE I. 



Haliclystus octoradiatus. 



Fig. 1. Two individuals adhering in natural postures to a blade 

 of Zostera. n s. 



Fig. 2. Oral view : i s. Interradial septum ; g b. genital bands ; 

 m /. muscular fibres ; g. f. gastral filaments ; s. sperm 

 mass ; c c. colleto-cystophore ; t. group of tentacles, x 2f . 



Fig. 2a. Diagram of trans, section of body : in. mouth ; i s. inter- 

 radial septum ; p c. perradial chamber ; g b. genital bands. 



