320 CONTRIBUTIONS TO THE NATURAL HISTORY 



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differ, and this we see introduced only among the higher Echinoderms. For, though 

 bilateral symmetry can be recognized among star-fishes and Echini proper, their anterior 

 row does not yet differ, and the first appearance of such a difference is introduced in the 

 family of Clypeastri, and more developed in Spatangi. If, therefore, the Echinoderms, 

 which as a whole rank above Medusse, still retain so much of the radiated type, and 

 the bilateral symmetry is developed in them among so many of their types solely in 

 the perfect symmetry of right and left, without a difference between forwards and 

 backwards, why should we expect this still earlier in the class of Acalephte, especially 

 when we are able to refer this type so easily 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 refer- 

 ence 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 doubtful which is right and which is left ; 

 for the sides are so completely identical, the two angles of the mouth are so absolutely 

 equal, the prominent projections of the opposite area so uniform, as to aflford no in- 

 dication upon this point. This is a very remarkable circumstance to occur in a class 

 intermediate between two others, in which the anterior and posterior margin can 

 be fully ascertained in the radiated arrangement, even in the Polypi, though they rank 

 lower. Is there, however, not a compensation for this difference in the greater sym- 

 metry of the two sides, as there are only four rays upon which the development of the 

 animal takes place in Medusae, while in other Radiata the numbers are odd ? But, 

 again, among Polypi, in the family of Halcyonoids we have the tentacles strictly in 

 pairs, and here, also, the oblong opening of the mouth passes between the four pairs 

 of tentacles, in such a manner as to render the anterior and posterior extremities 

 absolutely equal. In this respect, Alcyonium would agree with Pleurobrachia, 

 and justify the position I have ascribed to its bilateral symmetry. In another 

 great type of the animal kingdom, we have a similar, though inverse case, in the 

 family of Brachiopods, in which the anterior and posterior extremities are perfectly 

 symmetrical, but in which the right and left are widely different, and a pair of ten- 

 tacles which, in some respects, might be compared to the tentacles of Pleurobrachia, 

 placed forwards and backwards. So that, if we were justified in taking our standard 

 of comparison from the arrangement of parts in another type, there could be no hesita- 

 tion in considering the greater diameter of Pleurobrachia as the antero-posterior diameter, 

 and one of the tentacles as the anterior and the other as the posterior, and the mouth 



