PRIMITIVE MULTICELLULAR ANIMALS 



choanocytes to the linings of the excurrent canals and the extension of the 

 dermal epithelium to line the entire spongocoel. The homologies between the 

 asconoid and syconoid types are clear, however. The third, or leuconoid, type 

 of canal system can be derived by folding of the wall of the syconoid type with 

 its two sets of canals, and by the development of extensive subdermal spaces 

 within which water circulates on its way to the flagellated chambers. The 

 most highly organized sponges have very elaborate canal systems and small, 

 spherical flagellated chambers, but all can be compared, in the manner in- 

 dicated, with the simple arrangement in the asconoid type. These higher 

 sponges are further complicated by increase in the number of oscula and 

 spongocoels, each the center of a canal system, and by the indefinite growth 

 of the entire mass. 



Fig. 9.9. Stages in the formation of a triradiate spicule by scleroblasts in a calcareous 

 sponge. Individual cells come together to form a "trio"; each cell then divides, and the 

 resulting "sextet" (.4) forms and finishes the spicule {B and C). Among the ceils of the 

 sextet, basal and apical cells may usually be distinguished. (Adapted from E. A. Minchin, 

 1898, Quarterly Journal of Microscopical Science, vol. 40.) 



277 



