LUCERNARI.E AND THEIR ALLIES. 27 



advocates of a distinctive nomenclature for the Acalepha?. The former designated 

 the walls of the body as the ''foundation membranes," and the latter applied to the 

 same, the distinctive names : ectoderm for the outer wall and endoderm for the inner 

 wall of Hydroida. Later Allman added several other names to what he considered 

 to be subdivisions of the ectoderm ; but, as his views in regard to the relation and 

 mode of development of the Avails of a medusoid are so widely at variance with our 

 own that we are wholly at a loss in the attempt to homologize the several parts of 

 Avhat we believe to be the typical medusa with those of Allman's type, we shall 

 merely refer the reader to the section (Pait XI) where these things are set forth 

 in full detail, and proceed to describe the matter in hand with such terms as we 

 may find most convenient and best adapted to our theory. These terms have been 

 already, in part, promulgated in a note to an article' on the non partlicnofjenesis 

 of Tubularia, and we shall add here a few more as the necessities of the case may 

 demand. It will be understood at a glance that, since we apply this nomenclature 

 to all of the Acalepha? — the Ctenophorae being excluded, as we believe them to 

 belong to a distinct class — we hold to the identity of the general conformation of 

 the organs of every order included in this group. The minor details which serve 

 • to characterize each order and distinguish it from every other, may be, in part at 

 least, indicated by the mode of using the nomenclature or by the introduction of 

 such combinations of terms as will suit the ever-shifting exigencies of descriptive 

 anatomy. For the sake of the convenience of reference, and a ready understanding 

 of these terms, we have so constructed an index that it may be used as such, and 

 at the same time for a tjlossaru, by referring to the numbered paragraphs in the 

 body of the memoir. 



(il. The Opsophragma. (PI. iii, fig. 33; PI. IT, figs. 44, 47, 47%- PI. r, figs. 

 53, 54, 60; PI. VI, figs. 61, 62, 63, 64; PI. vii, figs. 74, 77; PI. viii, figs. 85, 88, 

 90, 91, 93; n to n^.) — What one Avould very naturally call the outer wall of the 

 body (without any reference to its mode of formation, but simply because it covers 

 the organization from its extreme anterior end to its posterior terminus), in reality 

 embodies two distinct subdivisions ; yet both of them lie upon the surface. One 

 of these divisions extends from the mouth to the edge of the umbella, and the other 

 from the latter point to the posterior end of the peduncle. The first of these cor- 

 responds to what we have, on a former occasion, designated as the endo2)hragma 

 of the medusa-form of the Hydroida, on account of its internal position, within the 

 campanule, during the process of fissigcmniation. Under present circumstances, 

 however, we have deemed it best to introduce another term, of equally distinctive 

 meaning, but having particular reference to our views in regard to the antero- 

 posterior axis of the body — opsophragma, meaning the face-ioall. It is confined 

 strictly to the anterior division, ox front face, of the umbella, and embraces witliin 

 its folds the tentacles and the marginal adhesive corpuscles (anchors). It varies in 

 thickness to a considerable degree, and passes from the minimuni to the maximum 



' "Tuhulana -not Pnrfhenorfriiniis," Am. Jouni. Science, .Jan. 1804, p. 05. "On (lie walls of 

 the most hi}?lilj developed medusoid." 



