204 ESSAY ON CLASSIFICATION 



connect the Ctenophoras with the genuine Medusae, and an embryo- 

 logical 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 

 Halcyonoids, as limited by Dana. 



2d Class: Acalepha; with the following orders: Hydroids including 

 Siphonophorae) Discophoras, and CtenophonT. 



3d Class: Echinoderms; with Crinoids, Asteroids, 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 Cepha- 

 lopods, 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 

 with 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 of the branch of Mollusks. These differences, however, 

 constitute only class characters and exhibit in no way a different 

 plan. It is indeed by no means difficult to homologize all the systems 

 of organs of the Cephalopods with those of the other Mollusks, and 

 with this evidence the proof is also furnished that the Cephalopods 

 constitute only a class among the Mollusks. 



As to the differences in the development of the Cephalopods and 

 the other Mollusks, the type of Vertebrata teaches us that partial 

 and total segmentation of the yolk are not inconsistent with unity of 

 type, as the eggs of Mammalia and Cyclostomata undergo a total 

 segmentation, while the process of segmentation is more or less 

 limited in the other classes. In Birds, Reptiles, and Selachians the 

 segmentation is only superficial; in Batrachians and most Fishes it 

 is much deeper; and yet no one would venture to separate the Verte- 

 brata into several distinct branches on that account. With reference 



" Zoologische Brief e, I, 361. 



