MODERN SYSTEMS.

293

Since it can be shown that Echinoderms are, in ageneral way, homologous in their structure with Acalephsand Polypi, it must be admitted that these classes belongto one and the same great type, and that they are theonly representatives of the branch of Radiata, assumingof course that Bryozoa , Cor alii nas, Sponges, and all otherforeign admixtures, have been removed from amongPolyps. Now, it is this Cuvierian type of Radiata, thusfreed of all its heterogeneous elements, which Leuckartundertakes to divide into two branches, each of which heconsiders coequal with Worms , Articulates, Mollusks , andVertebrates. He was undoubtedly led to this exaggera-tion of the difference existing between Echinoderms onone side, and Acalephs and Polypi on the other, by theapparently greater resemblance of Medusae and Polypi , 1and perhaps still more by the fact, that so many genuineAcalephs, such as the Hydroids, including Tubularia,Sertularia, Campanularia, etc., are still comprised bymost zoologists in the class of Polypi.

But since the admirable investigations of J. Mullerhave made us familiar with the extraordinary metamor-phosis of Echinoderms , and since the Ctenophorm andthe Siphonophorse have also been more carefully studiedby Grube, Leuckart, Kolliker , Vogt, Gegenbaur , and my-self, the distance which seemed to separate Echinoderms from Acalephs disappears entirely, for it is no exaggera-tion to say, that, were the Pluteus-like forms of Eehino-derms not known to be an early stage in the transforma-tion of Echinoderms , they would find as natural a placeamong Ctenophorse, as the larvae of Insects among Worms .I therefore maintain, that Polypi, Acalephs, and Echino-

1 We see here clearly how the con- den the primary feature of branches,sideration of anatomical differences their plan, and exalted a class to thewhiehcharacterizeclasseshasovcrrid- rank of a branch.