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ESSAY ON CLASSIFICATION.

derms constitute one indivisible primary group of theanimal kingdom. The Polypoid character of youngMedusae proves this as plainly as the Medusoid characterof young Echinoderms .

Further, nothing can be more unnatural than the transferof Ctenophorae to the type of Mollusks which Yogt hasproposed, for Ctenophorse exhibit the closest homologywith the other Medusae, as I have shown in my paper onthe Beroid Medusae of Massachusetts . The Ctenophoroidcharacter of young Echinoderms establishes a secondconnection between Ctenophorae and the other Eadiata,of as great importance as the first. We have thus ananatomical link to connect the Ctenophorae with thegenuine Medusae, and an embryological link to connectthem with the Echinoderms .

The classification of Eadiata may therefore stand thus :

1st Class : Polypi; including two orders, the Actinioidsand the Halcyonioids, as limited by Dana.

2nd Class : Accdephcei with the following orders : Hy-droids, (including Siphonophorae,) Discopliorae, and Cteno-phorae.

3rd Class : Echinoderms ; with Crinoids, Asteriqids,Echinoids, and Holothurioids, as orders.

The natural limits of the branch of Mollusks are easilydetermined. Since the Cirripeds have been removed tothe branch of Articulata, naturalists have generally agreedto consider, with Cuvier , the Cephalopods , Pteropods,Gasteropods , and Acephala as forming the hulk of thistype, and the discrepancies between modern investigatorshave mainly resulted from the views they have takenrespecting the Bryozoa , which some still consider asPolyps, while others would unite them with the Worms,though their affinity with the Mollusks seems to me to