Chap. I. STRUCTURAL FEATURES. 159 



the other longitudinal or antero-posterior. This question can only bo answered 

 in connection with those features in the structure of cei'tain Radiates which exhibit 

 more distinct traces of bilateral symmetry than the Ctenophora^, and in which right 

 and left become unmistakable in consequence of the presence of an odd spherosome. 

 Such comliinations exist among the Echinodenns, in which, in addition to two or 

 more pairs of spheromeres, there is an odd one, in the direction of a plane passing 

 through the ojiposite ends of the alimentary canal. And now, wlien it is con- 

 sidered that the tendency of the digestive tube to open in an excentric position 

 coincides with the elongation of the body of Echinoderms, and that the anus is 

 farthest removed from the mouth in those Spatangoids in which bilateral symmetry 

 is most sti'ikingly blended with radiation ; when it is further considered, that in 

 these animals the odd ambulacral zone coincides with the diameter along which 

 the mouth and anus are placed, at opposite ends of the body, and that the sym- 

 metrical pairs of amljulacral zones are placed on ojsposite sides of that longitudinal 

 diameter, — the conclusion seems irresistible, that the flattening of the digestive 

 cavity of the Ctenophora; in the direction of the longer diameter of the actinostome 

 is the first indication among Acalephs of a tendency to form an alimentary canal 

 in the direction of the longitudinal diameter of their body, and that the additional 

 radiating tubes must be lateral. 



Another consideration seems to militate in favor of that view. Admitting the 

 general homology of the radiating chyiiiiferous tubes with the ambulacral system 

 of the Echinoderms, and that there are as many sj^heromeres in the body of 

 Radiates as there are main branches of these systems, it must be aj^parent, that 

 while the majority of Echinoderms have five siiheromeres, the Ctenophora^ have 

 eight; that in Echinoderms there are two pairs and an odd one, and in Cte- 

 nophortB four pairs ; and that, therefore, the zones alternating with the and:)ulacra 

 in Echinoderms form also two pairs, with an odd interambulacral zone opposite the 

 odd ambulacral zone, while in Ctenophorai there are four pairs of zones homologous 

 to the ambulacra, and four pairs homologous to the interambulacral zones. If, 

 therefore, the diameter passing through the intermediate chjaniferous tubes Avere 

 considered as the antero-posterior diameter, there would be identical zones, that is, 

 interamljulacral zones, at both ends of that diameter; while in Echinoderms, the 

 zone at one end of the longitudinal diameter differs from that at the other end, 

 one being ambulacral and the other interambulacral. Now it is true, as there is 

 no odd zone in Ctenophora?, it may seem indifferent to consider either of their 

 interamlKdacral zones in the direction of the transverse diameters as the anterior 

 and posterior or the lateral ones ; but if there is no odd zone, there is at least 

 a substantial reason for considering the diameter which coincides with the longer 

 diameter of the actinostome as the antero-posterior diameter, namely, the fact that 



