CONTENTS. IX 



CHAPTER XXXIII. CEPHALOPODA continued. PAGK 



3. The development of the external form of the embryo . . 250 



A. Development of the embryonic rudiment through its 



extension over the greater part of the egg with subse- 

 quent development of a yolk-sac ..... 252 



(a) Loligo 252 



(b) Octopus 264 



(c) Argonauta 266 



B. Development without actual yolk-sac . . . . 267 



C. Development through the formation of the embryonic 

 . rudiment on a limited portion of the egg vrith simul- 

 taneous development of a large yolk-sac . . . 272 



4. The further differentiation of the germ-layers and the forma- 



tion of the organs 277 



A. The separation of the germ-layers and the formation of 



the yolk-epithelium and the alimentary canal . . 277 



B. The covering of the body and the shell .... 286 



The interpretation of the shell in recent Cephalopods, 



and comparison with fossil forms . . . . 287 



C. The sensory organs . 295 



The olfactory organs 296 



The otocysts . . . . . . . .296 



The eyes 298 



D. The nervous system 301 



E. The cartilaginous skeleton 305 



F. The gills . . . . 305 



G. The mesodermal structures 306 



Chromatophores, subcutaneous tissue, musculature . 309 



Literature 310 



CHAPTER XXXIV. GENERAL CONSIDERATIONS ON THE 



MOLLUSCA ..315 



Cleavage and formation of the germ-layers .... 315 



Larval form (Trochophore) ....... 318 



Derivation of the Mollusca 320 



The shell and the foot 322 



Origin of the organs 327 



Literature 332 



CHAPTER XXXV. TUNICATA. By K. HEIDER 334 



I. Sexual reproduction 334 



1. Larvacea (Appendicularia) 334 



2. Ascidiae Simplices and Compositae ...... 334 



A. Oviposition, fertilisation and egg-envelopes . . . 334 



B. Cleavage 338 



C. Formation of the germ-layers. Appearance of the 



medullary tube and the notochord .... 341 



D. Development of the free-swimming larva . . . 354 



External form of the body . . . . . 354 

 b 



