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ESSAY ON CLASSIFICATION.

The fundamental groups adopted by Vogt 1 are : Pro-tozoa, Radiata, Vermes, Mollusca , Cephalopoda , Articulata,and Vertebrata , an arrangement which is based solelyupon the relations of the embryo to the yolk, or the ab-sence of eggs. But, as I have already stated, this is anentirely unphysiological principle, inasmuch as it assumesa contrast between the yolk and the embryo within limitswhich do not exist in nature. The Mammalia , for in-stance, which are placed, like all other Vertebrata , in thecategory of the animals in which there is an oppositionbetween the embryo and the yolk, are as much formed ofthe whole yolk as the Echinoderms or Mollusks . The yolkundergoes a complete segmentation in the Mammalia , aswell as in the Radiates, the Worms and most Mollusks ; andthe embryo, when it makes its appearance, no more standsout from the yolk than the little Starfish stands out fromits yolk. These simple facts, known since Sars andBisehoff published their first observations twenty yearsago, is in itself sufficient to show that the whole principleof classification of Vogt is radically wrong.

Respecting the assertion, that neither Infusoria nor

hibiting radiating partitions, project-,ing inward from the outer wall ofthe body into the main cavity, andin having a digestive cavity derivedfrom the inversion of the upper partof that wall into the upper part ofthe main cavity. In Acalephs thereare no radiating partitions, and thedigestive cavity is hollowed out ofthe mass of the body; the centralprolongation of the body rising abovethe digestive cavity in the shape oforal appendages, which are neverhollow as the tentacles of the Polypsare. The mouth tentacles of Cerian-thus, which are hollow, are not ho-mologous to the oral appendages ofthe Acalephs, but constitute only aninner row of tentacles, of the same

kind as those that project around theupper margin of the main cavity.Again, the marginal tentacles of theAcalephs are homologous to those ofthe Polyps, while their oral append-ages are characteristic of their class.I may add also that the radiatingpartitions of the Rugosa, which I re-fer to the Acalephs, as well as theTabulata, are not homologous to theradiating partitions of the Actinoids and Ilalcyonoids, but correspond tothe ridges of the stem of certain Hal-cyonoids, and are, like them, a footsecretion.

1 Vogt (Carl ), ZoologischeBriefe.Naturgeschichte der lebenden unduntergegangenen Thiere. Frankfurta M., 1851; vol. i, p. 70.