10 CTENOPHOR.E. 



extremity from the eye the first trace of a small cavity (the digestive 

 cavity of the adult), which increases in size till it becomes spherical. At 

 about this time there is found, between the four clusters of the locomo- 

 tive llapj)ers, a second cavity, which has at first no connection whatever 

 with the digestive cavity, and develops independently of it. This sec- 

 ond formed canity, now a large rectangular bag, slightly lobed between 

 each of the four clusters of locomotive flappers, is the chymiferous 

 cavity, from which the funnel and the chymiferous tubes take their 

 origin in somewhat older stages. With advancing age the walls of the 

 two cavities become more circumscribed, and at the same time more 

 clearly defined, approaching each other constantly, until finally they 

 open into each other. The digestive cavity and the chymiferous tubes 

 diminish in diameter, becoming more circumscribed, and losing little by 

 httle the character of broad pouches for that of narrow tubes, extending 

 through the gelatinous mass. The locomotive flappers extend with the 

 chymiferous tubes along each one of the four pouches, which have 

 given rise to two chymiferous tubes, one long and one short one, devel- 

 oping independently. This difference is barely perceptible in the adult 

 Pleurobrachia ; it is well marked in Mertensia, still better in Idyia, 

 quite promment in Lesueuria, and takes its greatest development in 

 Bohna, where adjoining tubes anastomose after almost endless windings 

 through the large lobes formed by the lateral projections of the gelati- 

 nous mass. The cause of the predominence of some of the sphero- 

 meres over the others, is the unequal development of these two sets of 

 tubes, which may or may not extend mto lobes, thus giving to the 

 Ctenophoroe the appearance of bilateral animals. But examine this 

 same development in another class of Radiates, among the Echino- 

 denns, in the Spatangoids, for instance, where the odd ambulacrum is 

 the one which takes the least development, when the other four are 

 more equally developed, and no one will for that reason forget their 

 radiate character, and call them strictly bilateral animals. 



We can thus distinguish, among Spatangoids, an anterior and a poste- 

 rior extremity, a right and a left side. In Ctenophorse, owing to the 

 peculiar manner in which the difference between the chymiferous tubes 

 is developed, we are enabled to distinguish simply two diameters, but 

 not an anterior and a posterior extremity, or a right and a left side ; it 

 seems, therefore, scarcely logical to call these animals bilateral, when in 

 reaUty they show less sign of bilaterality than the SjDatangoids, which 

 no one, except Huxley, seems to doubt belong to Radiates.* The axes 

 we can thus distinguish among the Ctenophorae by the unequal devel- 

 opment of the chymiferous tubes, would not enable us to decide whether 

 the long tubes of the different genera were the same tubes developed 

 more fully in the different species. For instance, we should at first 



* See analysis of this view in Agassiz's Contributions, Vol. V. p. 60, by A. Agasslz. 



