CONTENTS. XV 



nomenclature and the present usage The classification of Cuvier, 201. 

 Uniformity of usage of specific and generic names, 202. Selection of 

 a standard classification Differences of opinion regarding the rank of 

 the characters Claus' and Sedgwick's definitions of the nine branches 

 of the animal kingdom Protozoa Ccelenterata, 203. Echinoder- 

 mata Vermes Arthropoda Molluscoidea Mollusca Tunicata 

 Vertebrata, 204. The classes of importance in Paleontology and 

 their known range in geological time Species and genera of chief 

 use in tracing the history of organisms, 205. The classes of the 

 Animal Kingdom and their geological range, 206. Species of the 

 paleontologist Varieties Mutations The history of organisms; the 

 two methods of its study, 207. Embryos or fossils; the imperfection 

 of the evidence, 208. Mature individuals, not embryos, used by the 

 Paleontologist Differentiation attained during the first or Cambrian 

 Era, 209. Nature and extent of the elaborations Recurrence of char- 

 acters accounted for by descent, 211. Modern zoology applicable to 

 the fauna of the Cambrian Era Characters whose origin is traced 

 back to Cambrian time Protozoa, Metazoa Echinodermata Anne- 

 lids Arthropoda, 212. Insignificance of characters of marine inverte- 

 brates evolved since Cambrian time, 218. 



CHAPTER XII. 



THE TYPES OF CONSTRUCTION IN THE ANIMAL KINGDOM. 



Records of evolution expressed chiefly in generic and specific characters 

 Course of individual development supposed to have been constant, 

 219. Beginning of individual life and development Hypotheses re- 

 garding the phylogenetic evolution of races, 220. The undifferentiated 

 cell, 221. Polarity Antimeres and Metameres Radiate structure, 

 bilateral symmetry, and actinimeres Primary axis, 222. Somites, 

 arthromeres, and diarthromeres of the Arthropods, 224. Distinc- 

 tive characters of the Metazoa Molluscan type of structure De- 

 velopment of organs and their taxonomic rank and value, 225. The 

 principle of Cephalization, 226. Cephalization one of the expressions 

 of the general law of differentiation Meaning of homology and ho- 

 mologous parts Analogy and analogous parts, 227. Differentiation 

 illustrated in the case of motor organs Two directions in which differ- 

 entiation proceeds Ciliary motion, 228. Water-vascular system of 

 Echinoderms Cilia in Molluscoidea and Mollusca Skeletal parts 

 Multiplication of like parts preceding specialization of their functions, 

 229. Comparison between embryonic development and succession of 

 ancestors Muscular motion or specialized motion, and locomotion, 

 230. Differentiation of nervous system a concomitant of locomotion, 

 231. Differentiation along the digestive tract Differentiation of the 

 motory system into muscular and skeletal organs, 232. Archetypal 

 structure Cuvier's classification, 233. Von Baer's embryological 

 classification Fundamental divisions of classification discerned by 

 earlier naturalists, 234. The polymeric type The dimeric and mono- 

 meric types, 235. The metameric and diarthromeric types Meaning 

 of typical structures and types in modern Zoology, 236. 



