40 ACvVLEPIIS AS A CLASS. Part I. 



this boily wall, and not the mouth, wliich is surrounded by radiating ap^iendages 

 outside of the central opening. 



This comparison is in itself sufficient to show, that, while every part in a 

 Medusa, is homologous to every part in an Actinia, these homologies are only 

 general homologies; that is, indications that these two animals l)elong to the same 

 branch of the animal kingdom, Init that special homologies cann(.)t be traced 

 between them. The body of the Actinia has a tlat disk at the lower end, which, 

 though contiguous with the outer Avail, diflers from it so much as to constitute 

 a base of attachment entirely wanting in the Medusa. Tlie np})er part of the 

 lateral walls of the Actini;i is thinned in a manner which forms a sort of cir- 

 cular neck below the fringes, ficilitating the inversion of the whole margin towards 

 the centre in a manner impossil)le to the Medusa. The central ojjening of the 

 Actinia is not circumscriljed by the margin of the uj)})er part of the walls of the 

 body, but that margin is turned inward ; while in the ^ledusa it hangs free, 

 outward. The radiating hoUoAv spaces are limited in Actinia by radiating partitions, 

 the inner margin of which is free, and suspended vertically in the main cavity ; 

 while in the Medusa, what may l)e compared to the paititions of the Actinia is 

 a. continuous gelatinous mass, between whicli simjjle tuljcs are left, communicating 

 only through narrow openings Avith the central cavity, or in other words, the 

 homologous parts of the Actinia and Medusa exhil)it a, structure special to each. 



We find the same special homologies in all the Actinoid Polyps. They all have 

 a cylindrical body with a central cavity, divided into chandlers by radiating par- 

 titions, marginal fringes communicating with these chandters, and a digestive cavity 

 hanging free into that cavity beloAV the central opening ; Avliile in all Medusa^ 

 we find the same continuous gelatinous Ijody Avith a simple central cavity and 

 radiating tubes, and a margin of the central opening tmnied outAv;n-d. Xoav, if the 

 classes of each branch of the animal kingdom, as has ])ecn shoAvn in the first 

 volume of this Avork, are natural di\'isions exhibiting the same 2)lan of structin'e, 

 Imt liuilt respectively in diflerent Avays and Avitli different means, avc have, in 

 Actinia and in Medusa, the types of tAVO thstinct classes ; and it only remains for us 

 to examine Avhat are the natural limits of these classes, and Avliat diflerent kinds 

 of animals belong to each. I hold, hoAVCA'cr, that the })receding remarks are in 

 themselves sufficient to shoAV that it is an exaggeration of their affinities to imite, 

 as Leuckart has done, and as most German naturalists now do, the Polyj^s and 

 Acalephs in one and the same great division luider the name of Civlenterata.^ 



^ Leuckart (R.), Uvber dii: Mi)rjihuloj.'ie, iiml TIiIl-it, 15raiuiscli\vfig, 1848, Svo. j). 1.3. Full of 

 (lie Verwandtscliafls-VerliUltnisse il(_'r wiilii/lloseii oriijinal iiivestimtions and suggestion;^. 



