XIV CONTENTS 



PAGB 



V. Hirudinea 318 



Oviposition (cocoons) 318 



1. Cleavage ; formation of the germ-layers and estab- 



lishment of the external form of the body 



(Ehyncobdellid«) 319 



Formation of the germ band 321 



Development of the embryo 323 



Gnathobdellidae : cleavage ; germ bands . . 325 



2. Larvse of the Gnathobdellidse (ciliation and primi- 



tive kidneys) 327 



3. The further development of the body ; establish- 



ment of head and trunk 329 



4. Formation of the organs 331 



General considerations on the development and 



interpretation of the Hirudinea . . . 386 



VI. Branchiobdella 336 



Systematic position ....... 336 



Oviposition ; cleavage ; formation of the germ- 

 layers, etc 337 



General considerations regarding Annelida : larval 



forms (Trochophore) ...... 341 



Trochosphsera, Pilidium, and other larvae : descent . 342 



Head and trunk ; segmentation 346 



Literature 350 



Chapter XI. Sipunculid^. By E. Ivorschelt .... 357 



1. The development of Sipunculus (blastula, formation of 



the germ-layers) 357 



Development of the embryo 358 



The larva of Sipunculus ....... 361 



Metamorphosis into the adult animal .... 363 



2. The development of Phascolosoma 364 



General considerations : Sipunculidae and Echiuridse as 



Gephyreans; relationships to the Annelida . . 365 



Literature .......... 366 



Chapter XII. Ch.etognatha. By K. Heider .... 367 

 Systematic position ; oviposition ; cleavage ; formation of the 



germ-layers . 367 



Further development 370 



General considerations : relationships to the Annelida ; 



absence of the larva ; coelomic sacs ..... 371 



Literature 371 



Chapter XIII. Enteropneusta. By E. Ivorschelt . . . 373 



Anatomy ........... 373 



Development without Tornaria larva: cleavage; formation of 



the germ-layers; development of the embryo; larva . . 379 



Development by means of the Tornaria 381 



