294 ESSAY ON CLASSIFICATION. 



derms constitute one indivisible primary group of the 

 animal kingdom. The Polypoid character of young 

 Medusae proves this as plainly as the Medusoid character 

 of young Echinoderms. 



Further, nothing can be more unnatural than the transfer 

 of Ctenophorse to the type of Mollusks which Vogt has 

 proposed, for Ctenophorse exhibit the closest homology 

 with the other Medusoe, as I have shown in my paper on 

 the Beroid Medusoe of Massachusetts. The Ctenophoroid 

 character of young Echinoderms establishes a second 

 connection between Ctenophorse and the other Radiata, 

 of as great importance as the first. We have thus an 

 anatomical link to connect the Ctenophorse with the 

 genuine Medusoe, and an cmbryological link to connect 

 them with the Echinoderms. 



The classification of Radiata may therefore stand thus : 



1st Class : Polypi; including two orders, the Actinioids 

 and the Halcyonioids, as limited by Dana. 



2nd Class : Accdephce; with the following orders : Hy- 

 droids, (including Siphonophorse,) Discophoree, and Cteno- 

 phoroe. 



3rd Class : Echinoderms; with Crinoids, Asterioids, 

 Echinoids, and Holothurioids, 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, 

 Gasteropoda, 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 still consider as 

 Polyps, while others would unite them with the Worms, 

 though their affinity with the Mollusks seems to me to 



