GENERAL TNDEX. 



CHAPTER XVIII. 



POLYZOA. 

 BRYOZOA(EHRENB.); CILIOBRACHIATE POLYPI (FARRE). 



POLYZOA General survey of the class 



Anatomy of Bowerbankia densa 464 



Structure of tentacula 464 



Action of the tentacular cilia 464 



Power of seizing food 464 



Alimentary apparatus of the Polyzoa 465 



Process of digestion 465 



Structure of the cell of Bowerbankia 466 



Mechanism of setse around its orifice 466 



Muscular system of Bowerbankia 467 



FLUSTR.E and ESCHAR/E 468 



Arrangement of the polyp-cells 468 



Their structure 468 



Operculum 469 



Structure of tegumentary sac 469 



Changes of form at different ages 469 



Indications of vital action 469 



Anatomical structure of Flustra and Eschara 470 



Arrangement of their muscular system 470 



digestive apparatus . . 470 



Mechanism of opercular lid 470 



Nervous system of Flustrce .... 471 



Organs of sense 471 



Propagation of the Polyzoa 471 



by gemmation 471 



Gemmiparous reproduction of Pedicellina 471 



Laguncula 471 



Reproduction by true ova 472 



Arrangement of male apparatus 472 



Spermatozoa 473 



Structure of ova 473 



Impregnation and escape of mature ova 473 



Ova of Pedicellina 473 



Division of vitellus 473 



Development of embryo 474 



Growth of mature embryo 474 



Reproduction by ciliated gemmules 474 



Propagation of Halodactylus diaphanus 474 



FLUVIATILE POLYZOA 475 



Their general history 475 



Structure of external envelope 475 



Anatomy of Cristatella mucedo 475 



Arrangement of the tentacula in different genera ....... 476 



Structure of their alimentary canal 476 



Circulatory system replaced by ciliary action 477 



Respiratory function 477 



Reproduction of the Fresh- water Polyzoa 477 



Sexes frequently conjoined, sometimes separate 478 



Ova of two kinds 478 



Two modes of embryogenic development 478 



