Chap. II. LIMITS OF THE CLASS. 103 



parts are strictly homologous to those of the Acalephs, they bear no resemblance 

 to those of Mollusks. I have purposely selected, for this comparison, one of 

 the Ctenophora? in which the bilateral symmetry is most prominent, that the 

 bilateral appearance may not seem intentionally lessened. In our Bolina alata 

 seen from above {^Fig. 49), there appear eight rays, diverging from the centre : 

 these are formed by the eight rows of locomotive flappers which extend, like 

 meridians, upon the sides of the body. Under these flappers extend eight chyni- 

 iferous tubes, which are symmetrically radiate in their arrangement, like the ambu- 

 lacral rows upon the sides of a Sea-urchin. But this is not all. These tubes are 

 also homologous to the radiating chymiferous tubes of the ordinary Medusae, and 

 to the amljulacral system of the Echinoderms ; and while they bear only a general 

 homology to the latter, they have the most special homology to the radiating tubes 

 of the Medusas. In both they arise from the main cavity of the body; in both 

 they diverge from that centre towards the periphery ; in both they connect through 

 anastomoses at the periphery ; in both they carry the nutritive fluids to all parts 

 of the body ; in both they are accompanied by the sexual organs ; while neither 

 Brj-ozoa nor Tunicata, nor any other Mollusks, have such radiating tubes. In the 

 Ctenophoraj, as in the other Acalephs, the digestive cavity is hollowed out of the 

 mass of the body, with the single difference that the abactinal end of that cavity 

 is freed from the spherosome in Ctenophorai, while it is not so in the other 

 Acalephs. The Bryozoa and Tunicata, on the contrary, have a distinct alimentary 

 canal entirely free from the walls of the body, and provided with two openings; 

 besides which Tunicata have a heart and a gill, and muscular bundles arranged 

 symmetrically upon the two sides of the body. Fuller evidence of the liilateral 

 structure of the Br3-ozoa and Tunicata, and of their typical difference from the 

 Ctenophoraj, could hardly be desii-ed. 



Assuming, then, that the Ctenophoras are genuine Radiates, it remains to be 

 seen whether they form a class by themselves, as not only Vogt, but also Leuckart 

 and Gegenbaur, will have it, or whether they are only members of the class of 

 Acalephs : for I hold that naturalists have no more right to please themselves in 

 the limitation of the classes, than in the hmitation of genera and species, or any 

 part of a systematic exposition of the relations of animals; and that their task 

 should simply consist in ascertaining, upon clearly defined principles, what nature 

 teaches us respecting these affinities. They may, no doubt, disagree in the appli- 

 cation of these principles ; but a purely arbitrary classification is no longer ad- 

 missible. The validity of every group proposed hereafter by an investigator must 

 be discussed before it is admitted or rejected ; and the principles upon which the 

 discussions are conducted will themselves become more precise, and be settled more 

 firmly by these discussions. 



