EXPLANATION OF PLATE X. [continued). 

 Fig. 

 16-18. Vol. i. p. 173. Sporocysts of a species oi Halichondria, that at Fig. 18 burst 

 and discharging minute granular spores, x 600. 



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

 tozoa, as figured by Ernst Haeckel, x 1600. 



20-30, Proterospongia H^ckeli, S. K., vol. i. p. 363. — A collared monad which 

 excretes and socially inhabits a common gelatinous matrix or " zoocytium " 

 resembling the cytoblastema 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 amoebiform 

 cytoblasts of a sponge-body ; i> d, examples with collars and flagella retracted, 

 dividing by transverse fission ; c c, normal zooids, with their collars con- 

 tracted ; J, spore-mass ; z z, hyaline mucilaginous zoocytium; 21, smaller 

 colony of eleven zooids only, those a.t a a a exhibiting an amoebiform 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 d 3 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 Salpiiigoeca 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 J, x 1000; 30, social colony viewed in longitudinal 

 optical section. 



