PRINCIPAL SYSTEMS OF ZOOLOGY 203 



be admitted that these classes belong to one and the same great type 

 and that they are the only representatives of the branch of Radiata, 

 assuming of course that Bryozoa, CoralliuEe, Sponges, and all other 

 foreign admixtures have been removed from among Polyps. Now 

 it is this Cuvierian type of Radiata, thus freed of all its hetero- 

 geneous elements, which Leuckart undertakes to divide into two 

 branches, each of which he considers coequal with Worms, Articu- 

 lates, Mollusks, and Vertebrates. He was undoubtedly led to this 

 exaggeration of the difference existing between Echinoderms on one 

 side and Acalephs and Polypi on the other, by the apparently greater 

 resemblance of Medusas and Polypi,*^ and perhaps still more by the 

 fact that so many genuine Acalephs, such as the Hydroids, including 

 Tubularia, Sertularia, Campanularia, etc., are still comprised by 

 most zoologists in the class of Polypi. 



But since the admirable investigations of J. Muller have made us 

 familiar with the extraordinary metamorphosis of Echinoderms, and 

 since the Ctenophorae and the Siphonophoras have also been more 

 carefully studied by Grube,^ Leuckart, Kolliker, Vogt, Gegenbaur,^ 

 and myself, the distance which seemed to separate Echinoderms from 

 Acalephs disappears entirely, for it is no exaggeration to say that 

 were the Pluteus-like forms of Echinoderms not known to be an early 

 stage in the transformation of Echinoderms, they would find as 

 natural a place among Ctenophoras, as the larvae of Insects among 

 Worms. I therefore maintain that Polypi, Acalephs, and Echinoderms 

 constitute one indivisible primary group of the animal kingdom. 

 The Polypoid character of yoimg Medusas proves this as plainly as 

 the Medusoid character of young Echinoderms. 



Further, nothing can be more unnatural than the transfer of 

 Ctenophoras to the type of Mollusks which Vogt has proposed, for 

 Ctenophoras exhibit the closest homology with the other Medusae, as 

 I have shown in my paper on the Beroid Medusae of Massachusetts. 

 The Ctenophoroid character of young Echinoderms establishes a 

 second connection between Ctenophorae and the other Radiata, of 

 as great importance as the first. We have thus an anatomical link to 



«We see here clearly how the consideration of anatomical differences which charac- 

 terize classes has overriden the primary feature of branches, their plan, to exalt a class 

 to the rank of a branch. 



' [Adolf E. Grube, 1812-1880.] 



« [Carl Gegenbaur, 1826-1903.] 



