84 



THE GRANTIA 



Exercise 3. — Microscopic Structure. 



(d) Stained sections of the grantia permit an examination of the 

 cellular structure; in preparing such material the calcareous spicules 

 are usually dissolved by acid to facilitate the sectioning. The cells of 



A^ / Ascon 



Leucori 



W/////////A 



■ic 



Fig. 41. — Canal systems of sponges. ^, ascon type. 5, sycon type. C, leucon 

 type. D, a portion of the wall of a sponge showing a specialization of the 

 sycon type. E, a portion of the wall of a sponge showing a specialization of 

 the leucon type. In the sycon or leucon types, the openings into the incur- 

 rent canals (t c) are called ostia, the openings from the excurrent canals 

 (e c) are called apopyles, while the openings from the incurrent to the 

 excurrent canals are called prosopyles and correspond to the pores (p) of 

 the ascon type; arrows indicate the direction of the water currents, c, cloaca; 

 e c, excurrent canal; i c, incurrent canal; os, osculum; p, pore. 



(Redrawn from E. A. Minchin, in E. R. Lankester, "Treatise on Zoologj," copyright, 

 1900, by A. and C. Black, printed by permission.) 



sponges are so delicate that good sections are difficult to obtain. 

 Covering the outer surface and lining the incurrent canals are the 

 pavement-like dermal cells. Lining the excurrent canals are the 

 collar-cells, or choanocytes; the collars and flagella of these may be 

 retracted, as well as shrunken in the preservation, and hence not easily 



