OF THE ACALEPHjE OF NORTH AMERICA. 319 



length of the axis of the body, and divide it into a right and left half, and are 

 therefore the tentacles lateral appendages, one on the right side and the other on 

 the left side, as we should consider them if we place the axis of the mouth in the 

 same direction as the axis of the mouth in Polypi, or have we to consider tiie ten- 

 tacles as arranged along the longitudinal axis, one on the anterior and the other on 

 the posterior extremity ? And in that case, is the split of the mouth rather the first 

 indication of an upper and lower lip, — as we should consider them were we to com- 

 pare the transverse position of the mouth with the position this opening assumes in the 

 oblong symmetrical Echinoderms, in which the bilateral symmetry has been made prom- 

 inent, — or have we to view also the indication of bilateral symmetry among Polypi 

 as a tendency to such an arrangement between the two lips ? 1 think I can be positive 

 in the case of Polypi ; for in Actinia, as well as in Astrangia, the oblong fold of the 

 mouth is unequal in its two angles ; and it were to suppose the right and left angle 

 of the mouth to be unequal, and the upper and lower lip to be symmetrical, if we 

 do not agree to consider this split as running in the longitudinal axis. And that it 

 indicates really a longitudinal axis is shown by the circumstance, that fecal matters 

 are discharged along the rounded angle of the oblong mouth, opposite to which 

 there is in many Polypi a tentacle of a peculiar form. But this being the case, 

 are tliere reasons to view Pleurobrachia in a different light? Are they really in 

 their arrangement more nearly related to Echinoderms than to Polypi ? I hardly 

 believe it ; for, as the mouth is transverse in so many Echini, their anterior and 

 posterior extremities always 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 Radiata in a position in 

 which the split of the mouth will indicate the antero-posterior diameter, even though 

 the diameter considered as the transverse be thus greater than the longitudinal. This is, 

 however, not the only instance of this case in the animal kingdom. In many Mollusca 

 of the class Acephala, in the family of Cardiacea and Brachiopoda, we have numbers of 

 genera and species in which the longitudinal axis is shorter than the transverse. And 

 though the vertical rows of locomotive fringes may remind us of the ambulacra of Echi- 

 noderms, I still hold that such a position as I assign to them is in more direct accordance 

 with the general progress of symmetry among Radiata than the reverse. The first 

 tendency beyond the pure radiated arrangement which is introduced among them is 

 to a symmetrical disposition and parity between right and left, even if the anterior and 

 posterior extremities be marked only by this lateral symmetry, and are not made to 

 differ from each other. Next, the two ends of the antero-posterior diameter are made to 



