198 ESSAY ON CLASSIFICATION 



The question which I would examine here in particular, is not 

 whether the circumscription of these great groups was accurately de- 

 fined by Cuvier, whether the minor groups referred to them truly 

 belong there or elsewhere, nor how far these divisions may be im- 

 proved within their respective limits, but whether there are four great 

 fundamental groups in the animal kingdom, based upon four differ- 

 ent plans of structure, and neither more nor less than four. This 

 question is very seasonable, since modern zoologists, and especially 

 Siebold, Leuckart, and Vogt have proposed combinations of the 

 classes of the animal kingdom into higher groups, differing essen- 

 tially from those of Cuvier. It is but justice to Leuckart to say that 

 he has exhibited, in the discussion of this subject, an acquaintance 

 with the whole range of Invertebrata,^ which demands a careful con- 

 sideration of the changes he proposes, as they are based upon a criti- 

 cal discrimination of differences of great value, though I think he 

 overrates their importance. The modifications introduced by Vogt, 

 on the contrary, appear to me to be based upon entirely unphysio- 



^ Ueber die Morphologic . . . der wirhellosen Thiere (1848). The readiness with 

 which the German naturalists have acquiesced in the proposition of Leuckart to unite 

 the Polyps and Acalephs into one class, seems to be owing to the circumstance that 

 their opportunities for studying the Polyps have been chiefly limited to the Actiniae. 

 Had they been able to extend their investigations to the Astrasans and Madrepores, 

 and to the many types of Halcyonoids which characterize the Faunae of the tropics, 

 they could not have failed to perceive that the Polyps constitute for themselves a 

 distinct class, foimded upon a special mode of execution of the plan which distinguishes 

 the Radiata from the other branches of the animal kingdom. Their investigations have 

 truly shown, what several French naturalists have long maintained, that many families 

 of Radiata, long referred to the class of Polyps, such as the Hydroids, cannot be 

 separated from the Acalephs; but they have been misled, by the evidence thus ob- 

 tained, to an exaggeration of the affinities of the Acalephs and Polyps. The Polyps, as 

 a class, differ from the Acalephs in exhibiting radiating partitions, projecting inward 

 from the outer wall of the body into the main cavity, and in having a digestive 

 cavity derived from the inversion of the upper part of that wall into the upper part 

 of the main cavity. In Acalephs there are no radiating partitions, and the digestive 

 cavity is hollowed out of the mass of the body; the central prolongation of the body 

 rising above the digestive cavity in the shape of oral appendages, which are never 

 hollow as the tentacles of the Polyps are. The mouth tentacles of Cerianthus, which 

 are hollow, are not homologous to the oral appendages of the Acalephs, but constitute 

 only an inner row of tentacles, of the same kind as those that project around the 

 upper margin of the main cavity. Again, the marginal tentacles of the Acalephs are 

 homologous to those of the Polyps, while their oral appendages are characteristic of 

 their class. I may add also that the radiating partitions of the Rugosa, which I refer to 

 the Acalephs, as well as the Tabulata, are not homologous to the radiating partitions 

 of the Actinoids and Halcyonoids, but correspond to the ridges of the stem of certain 

 Halcyonoids, and are, like them, a foot secretion. 



