Chap. I. SUB-ORDERS OF CTENOPIIOR^E. 183 



cover it entirely, when brought together by an antitropic movement, in the direction 

 of the antero-posterior diameter. The rehitive size of these lobes, as well as of 

 the auricles, at the actinal end of the lateral chymiferous tubes, varies greatly in 

 different representatives of this sub-order, but the lobes and auricles exist in all ; 

 in all there are four auricles, one to each lateral chymiferous tube ; and in all 

 there are only two lobes of the spherosome, each formed by the combination of 

 two spheromeres. The locomotive flappers are limited to the compact part of the 

 spherosome, the lateral ones being generally much shorter than the anterior and 

 posterior ones. In no Ctenophorse is the bilateral symmetry more prominently 

 marked than in the Mnemiida? ; and when the lobes of the spherosome are fully 

 extended, the coeliac diameter greatly exceeds the diacoeliac diameter. 



To these three sub-orders I would refer all the Ctenophoraj I know; and unless 

 Cestum proves to have marked structural peculiarities not described by the natural- 

 ists who had an opportunity of examining that genus, it must be referred to the 

 Cydippida3, with which Eschscholtz has already associated it. I believe, however, 

 that Cestum is likely to prove the type of a distinct sub-order, if the account 

 given by Eschscholtz and Mertens of its chymiferous tubes is at all correct. But 

 the circumstance that both have overlooked the coeliac tube, which exists in all 

 Ctenophora?, lessens my confidence in their description. The figure of Vogt is 

 still more defective, as it also omits the interambulacral tube which Eschscholtz 

 represents. The most striking difference between Cestum and the true Cydippidaj, 

 to which Eschscholtz refers this genus, consists, according to the figures I have 

 examined, in the trend of the tentacular sac, which in Cestum is in the direct 

 prolongation of the interambulacral tube, and opens on the actinal side of the 

 spherosome, while in Pleuroln-achia it is bent in the opposite direction and opens 

 on the abactinal side. There can be no question on that point. LeSueur, — who 

 has published the first figure of Cestum, — Eschscholtz, Mertens, and Vogt, all repre- 

 sent it in the same position, in illustrations which are not copied one from the 

 other, and even represent diflferent species. All the authors who have described 

 Pleurobrachia represent that ap^iaratus in this genus as I have myself seen it, 

 trending in the opposite direction, with the sole exception of Grant, who erroneously 

 represents it in a reverse position, in a paper specially intended to illustrate a 

 nervous system which is wanting. This mistake may show the importance of 

 studying minutely the symmetry of these animals. 



Another peculiarity of Cestum, mentioned by some naturalists who have observed 

 this genus, is the presence of only four rows of locomotive flappers along the 

 abactinal side of the elongated spherosome : this is the statement of Eschscholtz. 

 Vogt represents such flappers on both sides of the animal, and says expressly 

 (Zool. Briefe, p. 257) that they exist upon all the margins of the bod3^ 



