rOLYPIFERA. 465 



Medusce; and he does so " from the fact that the MedusidcB 

 of authors consists of two groups, the Naked-eyed (Gym- 

 -nophthalmatd) and the Covered-eyed {StegnnopJithalmata). 

 Some of which, the Aurelia, is but a derivative zooid form 

 of an animal, essentially resembling Lucernaria ; while so 

 far as regards the Naked-eyed Medusce, we have no evidence 

 that any genus except jEginopsis, is otlier than the repro- 

 ductive zooid of one of the Hydridce or Scrttdariadce. It 

 seems better, therefore, to avoid the term Medusce, as the 

 denomination of an ascertained group, reserving it merely 

 to denote the medusiform creatures of whose origin we 

 are ignorant, but whose structure entitles them to a pro- 

 visional place among the Lucernariadce. As our know- 

 ledge increases, we shall be able to arrange those Medusce 

 which are the zooids of Hydridce, Sertulariadoe, Diphydcej 

 or Physoplioridoe, under their respective groups, while the 

 rest will form the sub-sections of the Lucernariadce ; the 

 first consisting of such forms as Aurelia and Rhizostoma, 

 zooids developed by fission from fixed Lucernariadce; the 

 second consisting of such forms as jEyinopsis, free Lucer- 

 nariadce, developed at once from the ovum, without any 

 fixed state." 



The Actinozoa are those Coslenterata in which the 

 stomach is a sac suspended within and entirely distinct 

 from the body, from whose parietes it is separated by 

 a portion of the general cavity of the body, Avhich may 

 receive the special denomination of " perivisceral cavity. * 

 The stomach communicates freely by an inferior aperture 

 with the general cavity. A rough conception of the rela- 

 tions between the Actinozoa and the Hydrozoa may ho. 

 obtained by supposing the walls of the natatorial disc of a 

 Lucernaria to become united with those of its central 

 polype ; it would then become, to all intents and purposes, 

 an Actizozoon. As the Hydra is the type of the Hydro- 

 zoa, so the "Sea-anemone" {A ztinid) is considered to be the 

 type of the Actinozoa. As in the Hydrozoa, the body of 

 the Actinia is essentially composed of two layers, a super- 

 ficial layer, composed of polygonal cells, frequently de- 

 tached and renewed again, beneath which lies a granular 

 layer ; whilst in the deep dermal layer two sets of mus- 

 cular fibres are found,- -a superficial circular, and a deep 



H n 



