Chap. I. STRUCTURAL FEATURES. 161 



CtenophorjB has a vertical main axis, which is usually the greatest diameter of 

 the spherosome, and two distinct transverse diameters, one of which is to be con- 

 sidered as the antero-posterior or longitudinal, and the other as the transverse, 

 diameter proper, it will appear that the eight spheromeres are arranged in pairs 

 upon the sides of the longitudinal diameter, in such a manner that there are two 

 pairs upon the sides, one pair in front, and one pair behind, — one spheromere of 

 each pair being on one side, and the other on the other side, of the longitudinal 

 diameter. Now, in all Ctenophora3, the spheromeres, which as pairs correspond to 

 one another, are always equally developed and of exactly the same structure, the 

 same size, and the same form, balancing one another completely upon the two 

 sides of the body, so that the spherosome exhibits no trace of one-sidedness or 

 unequal bulging, as exists in some of the Acephala. But while this is so, so long 

 as we compare the spheromeres of one and the same pair with one another, the 

 symmetry of the spherosome assumes a very different aspect when we -extend the 

 comparison from one pair to another pair ; for while the anterior and the posterior 

 pairs are again identical in structure, size, and form, they balance one another in 

 opposite directions, and differ still more widely from the two lateral pairs, which 

 also balance one another in opposite directions. These differences may be carried 

 so far that the anterior and the posterior pair l:)alancing one another symmetrically 

 may be much more developed than the lateral pairs, and have a greatly modified 

 though homological structure. The natural consequence of this peculiar sj'mmetry 

 is, that the anterior and posterior surfacej? of the spherosome are exactly alike ; and 

 that therefore, notwithstanding the existence of a distinct antero-posterior diameter, 

 it is impossible to determine which is its anterior and which its posterior end. 

 For the same reason it is impossible to determine which is the right and which 

 is the left side, even though it cannot be doubted that there are two symmetrical 

 pairs of lateral spheromeres. We are, on that account, unable to distinguish the 

 regions of the body of the Ctenophora? with all the desirable precision, and shall 

 be obliged to designate four of the spheromeres as the lateral spheromeres, and 

 the four others as the anterior or postei-ior spheromeres ; remembering, however, 

 that one pair of the latter stands opposed to the other, while two single sphero- 

 meres belonging to different pairs are opposed to one another upon the sides. 



The intermediate or mterambulacral chymiferous tubes are always placed between 

 two lateral sphei'omeres. This is a truly remarkable feature of the Ctenojihorae, 

 unique among Acalephs, since all the other types of the class have aU their 

 spheromeres evenly balanced. "We shall see presentl}^, that this peculiarity stands 

 in direct relation to the general mode of branching of the chymiferous tubes. I 

 may, however, at once call attention to the bearing which this fact has upon the 

 whole symmetry of the Acalephs. In giving prominence to the sides, it renders the 



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