Chap. I. STRUCTURAL FEATURES. 157 



standing at various angles "svitli the direction of their movements, and, wliat is 

 still more perplexing, the actinal and abactinal poles of the axis are now turned 

 one way and now the opposite waj'. This variability of the motions of the 

 Acalephs renders it exceedingly difficult to describe their natural attitudes in a 

 manner which shall not conflict with their organic structure. When at rest, 

 Ctenophorai assume three different attitudes, peculiar to three different types of 

 the order : Bolina and allied genera stand in a \'ertical position, with the actinal 

 pole of their main axis downward ; Pleurobrachia and allied genera stand also in 

 a vertical position, but the actinal pole of their main axis is tiu-ned upward ; 

 Idya and allied genera, on the contrary, assume a nearly horizontal position, their 

 main axis slightly slanting, the actinal pole being lower in the water than the 

 abactinal pole. When moving onward, in whatever direction the motion may take 

 l^lace, whether it be straight forward, or upward or downward, or in a circuitous 

 course, these different types of Ctenophoraj retain the same general relation of 

 their main axis to the surrounding medium, that is, the actinal and the abactinal 

 poles are in the direction of the motion, the abactinal pole moving forward in 

 Bolina, Avliile in Pleurobrachia the actinal pole is turned forward and in Idyia 

 obliquely backward. Besides moving in these waj's by the more energetic action 

 of the whole spherosome, the Ctenophorte may change their position by the activity 

 of the locomotive flappers, when the main axis may assume any direction, accorchng 

 to the greater or less energj", or the total inactivity, of some of the rows of these 

 locomotive flappers, or of parts of one and the same row. In this way a slow 

 rotatory motion may also be produced, during which the main axis may, or may 

 not, change its direction. 



It is plain from these statements, that unless a nomenclature entirely irre- 

 spective of the various positions of these animals be adopted to describe their 

 structure and their movements, the strongest conflicts between the structural i-elations 



Echiuoderms differs only from that of the Acaleplis of Acalephs, and that they constilute one order, 



by its hardness, that the chyniiferons tnbes of the standing highest in that class. I cannot agree with 



Acalephs are homologons to the aquiferous system Leuckart and Geg(?nbaur, who consider them as 



of the Echinoderms, the marginal tentacles of the a distinct class ; nor does Gegenbaur's expression 



Acalephs homologous to the ambulacra! suckers of appear to me correct, -when he describes the di- 



the Echinoderms, etc. ; and once upon that track gestive cavity of the Ctenopborte as extending in 



it will be easy to embrace in these comparisons the longitudinal axis of the body, if what I have 



all the organic systems of all Eadiates, from the stated of the morphology of Acalephs in general 



simplest Polyp to the most highly organized Echi- is true: fur I hold that the main axis of all the 



noderm. As to the Ctenophorie in particular, I Radiates ought to be considered as tlie vertical 



have already shown (pp. 99-124) that they do not axis, around which the spheromeres are symmetri- 



forin a class by themselves, but belong to the class cally and radiatingly arranged. 



