Chap. III. GENUS PLEUROBRACIIIA. 211 



ment than to Polypi? I hardly believe it; for, as the mouth is transverse in many 

 Echini, so also do their anterior and posterior extremities differ more and more, 

 in the same proportion that the bilateral symmetry is increased and made more 

 prominent. It seems to me, therefore, more natural to compare Pleurobrachia with 

 the other Eadiata in a position in which the split of the mouth Avill indicate the 

 antero-posterior diameter, even though the diameter considered as the transverse be 

 thus greater than the longitudinal. This, however, is not the only instance of 

 such a disposition in the animal kingdom. In many Mollusca of the class of 

 Acephala, in the flimily of Cardiacea, we have numbers of genera and species in 

 which the longitudinal axis is shorter than the transverse. Though the vertical 

 chymiferous tubes with their rows of locomotive fringes are homologous with the 

 ambulacra of Echinoderms, I hold that the position I assign to the Ctenophorae 

 is in perfect accordance with the general progress of symmetry among Eadiata ; 

 for the anterior position of the mouth in the Spatangoids does not interfere with 

 its being the centre of radiation, as in all other Echinoderms. The first tendency, 

 beyond a perfectly radiated arrangement, which is introduced among the Radiates, 

 is to a symmetrical disposition and parity Isetween right and left, when the 

 anterior and posterior extremities may be mai'ked by this lateral symmetry only, and 

 not made to differ from each other. Next, the two ends of the antero-posterior 

 diameter are made to differ ; and this we see introduced among the higher Echi- 

 noderms only. For, though bilateral symmetry can be recognized among Star-fishes 

 and Echini proper, their anterior row does not yet differ from the others ; and the 

 fii'st appearance of such a difference is introduced in the Clypeastoids, and more 

 developed in the Spatangoids. If, therefore, the Echinoderms, which as a whole rank 

 above Medusae, still retain so completely the radiated type, and the bilateral sym- 

 metry is developed in them, among so many of their types, solely in their perfect 

 symmetry of right and left, without a difference between forward and backward, 

 why should we expect this in the class of Acalephje, especially when we are able so 

 easily to refer this type to that of Polypi ? I assume therefore decidedly, that the 

 diameter which corresponds to the split of the mouth indicates the longitudinal axis, 

 and shall, in the following pages, describe all parts with reference to this view. I 

 thus consider the halves of the body which would be divided by a plane passing 

 through the split of the mouth and through the opposite oblong area as the right 

 and left halves of this animal, and therefore the tentacles as being placed right 

 and left. But I must for the present leave it doulitful which is right and which 

 is left ; for the sides are so completely identical, the two angles of the mouth 

 so absolutely equal, and the prominent projections of the opposite area so uniform, 

 as to afford no indication upon this point. This is a very remarkable circumstance 

 to occur in a class intermediate between two others, in which, notwithstanding their 



