CTENOPHOKS;. 11 



sight suppose the long tii])es of Pleiirobraohia, of Mortonsia, of Irlyiii, of 

 BoUna, of Lesueuria, to be homologous, but such is not really the case ; 

 and the only means we have of determining this is the plane passing 

 through the tentacles, enabling us to ascertain whether the longitudinal 

 axis is in the trend, or at right angles to that plane. We shall soon see 

 that in Pleurobrachia and Mertonsia the plane, including the tentacles, 

 passes through the long axis, while in Bolina and Lesueuria it passes 

 through the short axis ; that the long tubes are on each side of the ten- 

 tacles, and consequently that the long tubes of Mertensia and the long 

 tubes of Bolina are not homologous ; but what corresponds strictly to 

 the lono- tubes of Pleurobrachia and Mertensia are the short tubes 

 placed on each side of the tentacular system. The lateral tubes invari- 

 ably hi the plane of the tentacular system give us the means of deter- 

 mining to which of these two classes Idyia belongs, and we find that 

 its longitudinal axis corresponds with that of Bolina, the lateral tubes 

 being in the shorter axis, as in the last-named genus, while in Pleuro- 

 brachia, as in Mertensia, they are in the longer axis. Such is the origin 

 of the characters which give to some members of the Ctenophorai their 

 remarkable bilateral appearance. It is simply a modification of what is 

 perfectly familiar to us among Echinoderms, and especially among the 

 Spatangoids ; but owing to the bilateral character of their develop- 

 ment, the CtenophoKV make us lose sight entirely of the original radi- 

 ate plan upon which these animals are built. Viewing, however, this 

 diflerentiation of the axis in all its stages, as we find it in Pleurobra- 

 chia, in Idyia, in Mertensia, in Lesueuria, in Bolina, we constantly keep 

 before our eyes the original formula from which the other members are 

 derived. 



Examined in the light of prophetic beings, the bilaterality of the 

 Acalephs is but another of those wonderful links which unite in one 

 great whole the difierent members of the Animal Kingdom. As the 

 Polyps are the prophetic representatives of the Acalephs in their em- 

 bryonic condition, the Hydroid state, so must we look at the Ctenophorie 

 as the prophetic type of those still more wonderful beings, the Echino- 

 derm lars'ae, in which bilateral symmetry is carried to such an extent 

 that even the great mind of a Miiller is led to consider them as exhibit- 

 ing a direct passage from a bilateral to a radiate plan of structure. In 

 the bilateral symmetry of the Ctenophoraj we are constantly reminded 

 of the general appearance of Echinoderm larvae, in which the radiate 

 structure should still be so far apparent as not to be concealed by the 

 bilateral symmetry. 



Looking at the Ctenophorae as prophetic animals, we are able to 

 undei'stand the separation of the digestive cavity into two distinct 

 parts. It is only what we find more fully developed in the Echiuo- 

 denn larvae ; the separation of a sort of alimentary canal, in Ctenophoras, 



