CONTENTS. XV 



PAGE 



B. hour-glass embryos, 279; C. acephalic and acaudal embryos, 280; D. final 

 stages of degeneration, 281; IV. Double embryos, 281. V. Triple embryos, 

 284. VI. Summary and conclusion, 287. 



CHAPTER XVI. 



THE DERMAL SKELETON 289-305 



The five kinds of skeletal structures in arthropods and vertebrates, 289. I. The 

 dermal skeleton of vertebrates, 289; the minute structure of the dermal bones in 

 the ostracoderms, 290; tremataspis, 290; pteraspis, 293; ateleaspis, 295. II. 

 Dermal skeleton of limulus, 296; minute structure, 297. III. Summary and com- 

 parison, 302. 



CHAPTER XVII. 



THE ENDOCRANIUM, BRANCHIAL AND NEURAL CARTILAGES 306-322 



I. The endoskeleton of arachnids, 306. II. The neural arches, 307. III. The 

 branchial cartilages, 307; development of branchial cartilages in limulus, 308; 

 minute structure, 309. IV. The endocranium, 312; apus, 312; mygale, 313; lim- 

 ulus, 314; scorpion, 317; telyphonus, 319. V. Summary and comparison, 319. 



CHAPTER XVIII. 



THE MIDDLE CORD, THE LEMMATOCHORD AND THE NOTOCHORD 323-336 



I. The middle cord of insects, 324; the lemmatochord of lepidoptera, 326. II. 

 The middle cord of the scorpion; A. neural sinus, merochord and bothroidal cord 

 of the adult, 328; B. development of the lemmatochord, 329; merochord, 330; 



C. development of the neural sinus, neuroglia and canalis centralis, 331. III. 

 The middle cord of limulus, 334. IV. Summary and comparison, 335. 



CHAPTER XIX. 



THE OSTRACODERMS AND THE MARINE ARACHNIDS 337~347 



Nature of the problem. I. The marine arachnids and their origin, 338. II. 

 The ostracoderms, 341; historical review, 342. 



CHAPTER XX. 



THE OSTRACODERMS 348-380 



Subdivisions of the body, 349; the cephalic appendages, 350; jaws, 350; skeleton, 

 351; trend of development of the exoskeleton, 352; the eyes, 355; olfactory organ, 

 356; auditory organ, 356; cutaneous sense organs, 356. I. Aspidacephali, 358; 

 cephalospidae, 358; trematospidae, 359; exoskeleton, oral region, lateral eyes, 

 marginal and postorbital openings, lateral line organs, appendages, ateleaspidse, 

 363. II. The anaspida, 364; ccelolepidae, birkeniidae. III. The pteraspida, 364; 

 pterospidae, psamostaedae. IV. Antiacha, 367; exoskeleton, 367; atrial frill, 371; 

 gills, 371; viscera, 372; jaws, 373; hyoid arches, 375; mouth, 375; eyes, olfactory 

 organs, sensory grooves, 376; cephalic appendages, 376; preservation, 377; loco- 

 motion, 379; food, 379. 



CHAPTER XXI. 



THE VERTEBRATES 381-392 



I. The cyclostomata, 383. II. The elasmobranchii and holocephali, 384. III. 

 The arthrodira, teleostomii dipnoi and amphibia, 386. 



