158 CTEN0PH0R.T3. Part II. 



of their jiiirts, and their attitiulcs, will lie unavoi(lal)lo, and the same motion in 

 two difleront types of this order might l)e designated hy contrary exi)ressions. 

 The only way of avoiding these difficulties is to adopt a nomenclature in accordance 

 with the general homologies of these animals, and to keep in view the fact that 

 the normal attitude of all the Kadiates is that in which their main axis is jilaced 

 in a vertical position. With this understanding Ave may then nay, that, when at 

 rest, Bolina and allied genera stand iipright, with the actinostome turned down- 

 ward ; that Pleurobrachia also stands upright, liut with the actinostoriie turned 

 upward ; that Idyia lies nearly horizontally, with the actinostome slanting slightly 

 downward ; and that Bolina and Idyia move witli the abactinal pole forward, while 

 Pleurol)rachia moves with the actinal pole forward. 



Next, we have to distinguish two other diameters, at right angles with one 

 another and with the main vertical axis. In order correctly to appreciate the 

 peculiar symmetry of the Ctenophora^ it must be remembered, in the first place, 

 that their body is made up of eight spheromeres, arranged in pairs on opposite 

 sides of an imaginary plane dividing the whole structure into equal halves, and 

 passing through the longer diameter of the circumscribed area of the abactinal 

 pole, as w^ell as through the longer diameter of the actinostome and of the digestive 

 cavity; and, in the second place, that there are two or more distinct radiating tubes, 

 opposite one another, and respectively^ intermediate l)etween two rows of locomotive 

 flappers, trending in the direction of another imaginary plane dividing also the whole 

 structure into equal halves, but at right angles with the first. Thus the body 

 of the Ctenophora^ may be divided into e((ual halves in two opposite directions; Imt 

 the greatest diameter of these two sets of halves is not equal : that which passes, 

 at right angles with the main axis, through the longer diameter of the actinostome 

 and of the circumscriljed area, is either greater or smaller than that which passes 

 through the two intermediate radiating tubes, and the preponderance of the one 

 over the other seems to be typical in difierent groups of Ctenophoriw In Idyia, 

 the transverse diameter passing through the intermediate radiating tulies is much 

 shorter than that which coincides ^vith the longer diameter of the actinostome ; 

 while in Pleurobrachia the relations are reversed, the transverse diameter passing 

 through the intermediate radiating tulles being longer than the other, except that 

 in this type the difference between the length of these two diameters is not so 

 marked. Bolina, again, coincides with Idyia as to the respective length of its 

 transverse diameters, and exhibits the same disproportion between the two. 



We have thus, unmistakably, two difierent kinds of transverse diameters, though 

 in different representatives of the order one or the other of the two kinds may 

 be respectively the longer or the shorter. This distinction once recognized, the 

 question arises how far one of these diameters may be considered as lateral, and 



