160 ESSAY ON CLASSIFICATION 



ized by a single system of organs, the intestine, they contain no dis- 

 tinct orders, but each class has three tribes, corresponding to the three 

 classes of this type, which are Infusoria, Polypi, and Acalephs. The 

 tribes of the class of Infusoria are Infusoria proper, Polypoid Infu- 

 soria, and Acalephoid Infusoria; the tribes of the class of Polypi are 

 Infusorial Polypi, Polypi proper, and Acalephoid Polypi; the tribes 

 of the class Acalephs are Infusorial Acalephs, Polypoid Acalephs, 

 and Acalephs proper. But the classes of Mollusks which are said to 

 be characterized by two systems of organs, the intestine and the vas- 

 cular system, contain each two orders, one corresponding to the In- 

 testinal animals, the other to the type of Mollusks, and so Acephala 

 are divided into the order of Gelatinous Acephala and that of Mol- 

 luscoid Acephala, and the Gasteropods and Cephalopods in the same 

 manner into two orders each. The Articulata are considered as repre- 

 senting three systems of organs, the intestinal, the vascular, and the 

 respiratory systems; hence their classes are divided each into three 

 orders. For instance, the Worms contain an order of Gelatinous 

 Worms, one of Molluscoid Worms, one of Annulate Worms, and 

 the same orders are adopted for Crustacea and Insects. Vertebrata 

 are said to represent five systems, the three lower ones being the in- 

 testine, the vessels, and the respiratory organs, the two higher the 

 flesh (that is, bones, muscles, and nerves) and the organs of senses; 

 hence, five orders in each class of this type, as, for example, Gelatinous 

 Fishes, Molluscoid Fishes, Entomoid Fishes, Carnal Fishes, and Sen- 

 sual Fishes, and so also in the class of Reptiles, Birds, and Mam- 

 malia.^^ 



I have entered into so many details upon these vagaries of the dis- 

 tinguished German philosopher, because these views, however crude, 

 have undoubtedly been suggested by a feature of the animal king- 

 dom which has thus far been too little studied: I mean the analogies 

 which exist among animals, besides their true affinities, and which 

 cross and blend, under modifications of strictly homological struc- 

 tures, other characters which are only analogical. But it seems to me 

 that the subject of analogies is too little known, the facts bearing 

 upon this kind of relationship being still too obscure to be taken as 

 the basis of such important gToups in the animal kingdom as the 



"See further developments upon this subject in Oken, Naturphilosophie, and Allge- 

 meine Naturgeschichte (14 vols., Stuttgart, 1833-1843), IV, 582. 



