202 GEOLOGICAL BIOLOGY. 



tion of the lower animals. He recognized four branches (Ani- 

 malia Vertebrata, Animalia Mollusca, Animalia Articulata, 

 Animalia Radiata). The first four classes of Linne's sys- 

 tem were united to form the first branch of Cuvier. The 

 most prominent character uniting them was the possession of 

 an internal skeleton, bound together by a segmented vertebral 

 column. The second branch of Cuvier, called Mollusca, in- 

 cluded six classes (Cephalopoda, Pteropoda, Gastropoda, AcepJi- 

 ala, Brachiopoda, Cirrhopodd), and the conspicuous charac- 

 ters of the Mollusca were the possession of a soft, bag-like 

 body, enclosed more or less completely by a hard exterior 

 shell composed of one, two, or more parts. Cuvier called the 

 third branch Articulata, including in it four classes (Annelida, 

 Crustacea, Arachnida, Insect a]. The chief character in this 

 branch was the segmented external skeleton, composed of 

 joints with lateral articulated appendages. The fourth branch 

 was Radiata, and included five classes (Echinoderms, Intestinal 

 Worms, Acalephcz, Polypi, Infusoria]. The prominent char- 

 acter was the radiate structure, typically exhibited in the Star- 

 fish or Sea-urchin, but ignorance of internal structure led to 

 the association of many unlike forms. Since Cuvier's time 

 great advance has been made in the knowledge of the struc- 

 tural anatomy of animals, especially in the smaller and lower 

 organisms, and many other classifications have been proposed, 

 but the majority of Cuvier's classes have remained. Animals 

 referred to some of the classes by Cuvier, and some newly- 

 discovered animals, have been made the types of other classes, 

 and stricter definitions of the classes already established have 

 been made. 



Uniformity of Usage of Specific and Generic Names. The 

 branches have been considerably remodelled, especially by 

 later zoologists, according as one or other organ or system 

 of organs has been taken as of chief importance in distin- 

 guishing the groups. Of the later classifications those of 

 Leuckhart, Huxley, Claus, Gegenbaur, and Lankester have 

 expressed new points of view in the arrangement of the or- 

 ganisms, but in all the confusion of systems a common usage 

 has grown up in the application of specific and generic names 

 to animals and plants, and these have constituted the standards. 



