CONTENTS. xill 



SECTION II. 

 Homology of Vertebrates. 



I. General Plan of Structure, or General Homology. — General 

 characteristics of vertebrates : (i) Relation of muscle to skeleton, (2) 

 possession of backbone, (3) possession of two trunk cavities, (4) struc- 

 ture of head, (5) only two pairs of limbs, strong suggestion of common 

 origin, 249. 



II. Special Homology. — Definition, the proof of common origin, best 

 shown in limbs, (a) Fore limbs of all classes compared, 250. Hind 

 limbs of different orders of mammals compared, 255. Plantigrade, 

 digitigrade, unguligrade, 256. Manus and pes, classification of ungu- 

 lates by foot structure, 257. Rudimentary and useless organs, 258. 

 Homology in other systems, 260. 



III. Serial Homology. — Definition of, serial homology of the verte- 

 brate skeleton, 261. A vertebral segment, 262. Owen's archetype, 

 modifications in the series, 263. Origin of limbs, 264. Serial homol- 

 ogy of other systems, 265. 



SECTION III. 

 Homology among Invertebrates. 



1. Articulata. — General plan of, 266. Serial homology, 267. So- 

 mite defined, repetition and modification of somites, 268. Illustrated 

 from crawfish, 269. Crab, modifications in going down the scale, 271. 

 In going up the scale, 272. Origin of insects' wings, 273. Law of 

 differentiation, homology of nervous system, 276. 



2. Mollusca. — General plan, explanation of, 277. 



3. Radiata. — General plan is radiated, 278. Comparison with 

 other types, 279. 



4. Protozoa, 279. 

 General conclusions, 280. 



PART II. 



ORGANS AND FUNCTIONS OF ORGANIC LIFE. 



CHAPTER I. 



NUTRITIVE FUNCTIONS — METABOLISM. 



Definition of metabolism, 283. Waste and its relation to life, to 

 work, to heat, 284. Necessity of food, 285. Necessity of waste- 

 removal, anabolism and katabolism, 286. Three divisions of the sub- 

 ject of this part, 287. 



