EXPLANATION OF PLATE X. (continued). 

 FIG. 



19. ASCETTA PRIMORDIALIS, Hkl. Three uniflagellate spores, or so-called sperma- 

 tozoa, as figured by Ernst Haeckel, x 1600. 



20-30. PROTOSPONGIA H^ECKELI, S. K. A collared monad which excretes and socially 

 inhabits a common gelatinous matrix or " zoocytium " resembling the cyto- 

 blastema of an ordinary sponge ; 20, a social colony of about forty monads, 

 x 800 ; at a a, zooids which, withdrawing their collars and flagella, have 

 assumed an aspect corresponding with the amcebiform cytoblasts of a sponge- 

 body ; b b, examples with collars and flagella retracted, dividing by transverse 

 fission ; cc, normal zooids, with their collars contracted ; s, spore-mass ; z z, 

 hyaline- mucilaginous zoocytium; 21, smaller colony of eleven zooids only, 

 those at a a a exhibiting an amcebiform aspect, and in one instance extending 

 slender pseudopodia ; 22, small social colony, including at a four spore-like 

 bodies produced by the subdivision of a metamorphosed collared zooid, and 

 at b b two minute, monadiform germs ; 23, small symmetrical colony-stock of 

 sixteen zooids, derived from the even and continued segmentation of a single 

 primary unit ; 24 and 25, still younger colonies of two and four zooids only ; 

 26, solitary attached monadiform germ, having as yet developed neither a 

 collar nor investing zoocytium, x 1000 ; 27, more advanced phase of the 

 same zooid, having a well-developed collar and mucilaginous investing sheath, 

 and corresponding at this stage with the earlier phases of Salping aeca ampulla, 

 S. K., represented at Plate III. Figs. 19 and 20 ; 28, metamorphosed collared 

 zooid, projecting a lobose extension of its anterior border beyond the margin 

 of the zoocytium ; 29, spore-mass, as at 21 s, x 1000 ; 30, social colony viewed 

 in longitudinal optical section. 



