44 



INVEETEBEATA 



CHAP. 



half lose their flagella and become granular, so that the blastula is 

 thus converted into an amphiblastula. The amphiblastula fixes 

 itself and undergoes a metamorphosis like that of Grantia, but the 

 resulting sponge, the "Rhagon," is conical not cylindrical, and the 



flagellated chambers are pro- 

 duced as hemispherical pouches 

 of the inner layer (Fig. 21). 



In the development of the 

 Tetractinellid Plakina, Maas 

 (1909) describes the larva as 

 beginning its free life as a 

 blastula, since the cells constitut- 

 ing its wall are at first all slender 

 and ciliated, but the blastocoele 

 contains a few rounded granular 

 cells, termed archaeocytes, 

 which seem to be the mother 

 cells of the germ cells. The 

 posterior half becomes granular 

 by the alteration of the cells, 

 which lose their cilia, but cells 

 which are not to be confused 

 with the archaeocytes are also 

 budded from this half into the 

 interior (Fig. 23). Fixation and 

 metamorphosis occur as usual, 

 but the resulting sponge has the 

 form of a shorter cylinder than 

 is the case with either Grantia 

 or Oscarella. By downgrowths 

 of dermal cells, the interior 

 flagellated cells become divided 

 into groups, which, although at 

 first they retain a portion of the 

 lumen of the sponge, eventually 

 become solid; from these solid 

 masses the spherical flagellated 

 chambers are formed later (Fig. 

 22). 



Finally, in the siliceous 

 sponge, Hsperia (Maas, 1892), 

 the larva is hatched as an amphi- 

 blastula, but the flagellated cells 

 cover four-fifths of the surface, 

 and the granular cells form a solid plug projecting into the interior 

 of the blastocoele and contain a sheaf of siliceous spicules, ready 

 for distribution throughout the tissues of the young sponge as soon 

 as fixation has occurred. Stretching across the blastocoele are 



Fig. 21. — Longitudinal sections through the 

 free-swimming larva of Oscarella lobularis 

 in two stages of its development and its 

 fixation. (After Maas, ) 



A, early larva. B, larva in which posterior cells 

 are beconjing granular. 0, Rhagon shortly after 

 fixation. Letters as in two previous figures. 



