PROTOZOA. 



13 



adult Sponge, without previous impregnation by a spermatozoid. Upon 

 this view, therefore, the development of the Sponge becomes capable of 

 being strictly paralleled by that of several groups of undoubted Protozoa, 

 but acquires a significance entirely different from that which must be 

 ascribed to the development of the Metazoa. 



ff 



Fig. 8. Structure of Spongida. A, Vertical section of the outer layer of Halisarca lobu- 

 laris, a Sponge in which the skeleton is wanting, enlarged 75 times (after F. E. Schultze) : 

 pp, "Pores," or openings of afferent canals by which water is conducted to the ciliated 

 chambers or " ampullaceous sacs "(a a); e, Commencement of a larger efferent canal, con- 

 ducting from the ampullaceous sacs to the deeper canals, by which the water is finally car- 

 ried off to be expelled from the "oscula;" g g, Young stages of the reproductive bodies or 

 spores. B, Part of a single ampullaceous sac of the same Sponge, transversely divided, and 

 enlarged 800 diameters (after Saville Kent), showing the flagellate monads or " sponge-par- 

 ticles," with their inwardly directed flagella. c, A single flagellate monad of the same, still 

 further enlarged : /, Flagellum ; m, Collar round the base of the flagellum ; n, Nucleus ; c, 

 Contractile vesicle. 



Accepting, then, in the meanwhile, the last-mentioned views as to the 

 real character and import of the development of the Spongida, it seems 

 inevitable that these organisms must, with our present knowledge, be in- 

 cluded among the Protozoa. This conclusion, moreover, is the one which 

 is clearly deducible from a study of the structure of the adult organism, 



