XVI CONTENTS. 



PART II. THE ACRANIATA. 



CHAPTER XXII. 



Page 



The Craniates and the Aceaniates ... . 393~4°7 



Statement of the problem, 393. I. The craniates, 395. II. The acraniates, 396; 

 metamerism, 398; appendages, nervous system, 399; degeneration, attachment, 

 mantle, 400; skeleton, heart and circulation, 401; sexual organs, development, 401; 

 A. molluscs and annelids, 403; trochosphere, gastrula, blastopore; B. craniates, 

 403; modification of the gastrula, telopore, concrescence; C. acraniates, 405; mes- 

 entocoel, telopore, mouth, 406; the naupula, ccelom, 407. 



CHAPTER XXIII. 



The Cirripeds, Tunicates, and Echinoderms 408-430 



I. The cirripeds, 408; the nauplius and the naupula, the metamorphosis, 410; 

 appendages, alimentary canal, 411; ccelom, excretory organs, 412; sexual organs, 

 degeneration, 413; the old mouth and the new. 11. The tunicates, 415; meta- 

 morphosis, heart and vascular system, 417; eyes, 418; the old mouth and the new, 

 mantle, 419; comparison with cirripeds, summary, 420. III. The echinoderms, 

 421; larva, 422; ciliated band, 423; cephalic appendages, 424; attachments, de- 

 velopment, 425; telocoel, mesoderm and ccelom, 426; thoracic appendages, 427; 

 excretory organs, disc, vertebrate, 428; asymmetry, summary, 430. 



CHAPTER XXIV. 



The Enteropneusta, Plerobranchia, Polyzoa-Brachiopoda Phoronida and 



Chaltognatha . . 431-453 



IV. The enteropneusta and echinoderms, 431; the enteropneusta and the arthro- 

 pods, 432; structure and development, 433; cleavage, 433 ; gastrula and telocoele, 

 433; mesoderm and ccelom, 434; metamorphosis, 435; cephalic caecum, 435; 

 late lairval and adult stages, 435; endocranium, 436; muscles, 437; ccelom, 437; 

 nervous system, 437 V. The pterobranchia, 439. VI. The polyzoa, 440; ento- 

 procta, 441; conclusion, 443; ectoprocta, 443. VII. The brachiopoda, 445. 

 VIII. The phoronida, 446; fusiform cells, 447. IX. The chsetognatha, 448; de- 

 velopment, 448; adult, 449; integument, 449; excretory organs, 449; trunk, 450; 

 head, 450; endocranium, 450; nervous system, 450; cephalic sense organs, 452; 

 lateral eyes, 452; parietal eye, 452; olfactory organ, 452; conclusion, 453. 



CHAPTER XXV. 



Summary and Conclusion 



I. The evolution of a creative environment, 454. II. Crises in organic evolution, 

 456; A. the evolution of metamerism and bilateral symmetry, 457; B. asymmetry 

 as a creative factor, 458; C. chiten and the exoskeleton as creative factors, 439; 

 D. the increasing volume of the yolk sphere as a creative factor, 461 ; E. the increas- 

 ing volume of the brain as a creative factor, 461; F. the creation of a new environ- 

 ment for the eyes, 462; the significance of a natural system of classification, 463; 

 the various aspects of evolution, 468. 



Explanation of the Lettering . " , .h-. 



Index ..... .483 



454-472 



