V 



184 ESSAY ON CLASSIFICATION. - Part I. 



But since the admirable investigations of J. Miiller have made us familiar with 

 the extraordinary metamorphosis of Echinoderms, and since the Ctenophoras and 

 the Siphonophoroe have also been more carefully studied by Grube, Leuckart, 

 Kcilliker, Vogt, Gegenbaur, and myself, the distance which seemed to separate Echino- 

 derms 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 

 Ctenophorse, as the larvas of Insects among Worms. I therefore maintain, that 

 Polypi, Acalephs, and Echinoderms constitute one indivisible primary gi'oup of the 

 animal kingdom. The Polypoid character of young Medusje 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 Ctenophorse exhibit the closest 

 homology with the other Medusas, as I have shown in my paper on the Beroid 

 Medusae of Massachusetts. The Ctenophoroid character of young Echinoderme 

 establishes a second connection between Ctenophorae and the other Radiata, of as 

 great importance as the fii-st. We have thus an anatomical link to connect the 

 Ctenophoroe with the genuine Medusae, and an embryological link to connect them 

 with the Echinoderms. 



The classification of Radiata may, therefore, stand thus : — 



1st Class : Polypi; including two orders, the Actinoids and the Halcyc- 

 noids, as limited by Dana. 



2d Class: Acalephae; with the following orders: Hydroids, (including Sipho- 

 nophorae,) Discophorae, and Ctenophorae. 



3d Class : Echinoderms; with Crinoids, Asteroids, Echinoids, and Holothu- 

 rioids, as orders. 



The natural limits of the branch of Mollusks are easily determined. Since the 

 Cirripeds have been removed to the branch of Articulata, naturalists have generally 

 agreed to consider, with Cuvier, the Cephalopods, Pteropods, Gasteropods, and 

 Acephala as forming the bulk of this type, and the discrepancies between modern 

 investigators have mainly resulted from the views they have taken respecting the 

 Bryozoa, which some consider still as Polyps, while others would unite them mth 

 the Worms, though their affinity with the Mollusks seems to me to have been 

 clearly demonstrated by the investigations of Milne-Edwards. Vogt is the only 

 naturalist who considers the Cephalopoda "as built upon a plan entirely peculiar;^ 

 though he does not show in what this peculiarity of plan consists, but only mentions 

 the well-known anatomical differences which distinguish them from the other classes 



* Vogt, (C.,) Zoologische Briefe, q. a. ; vol. 1, p. 3G1. 



