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TEXTBOOK OF ZOOLOGY 



spongioblasts are located in this layer. The incurrent canals are 

 lined with flat pavement cells. At each prosopyle there is a single 

 large dermal cell, a porocyte, which surrounds the aperture. In the 

 middle layer are found the reproductive cells and some amoeboid 

 wandering cells. The radial canals and the internal wall of the cen- 

 tral cavity have similar histological structure since the former are 

 outpouchings of the latter. The cells here are primarily special 

 shaped ehoanocytes, peculiar to sponges, interspersed with scat- 



Jpicule — 



Sckroblast 



. Dermal cells 



-flaciei 



lum 



collar 



AirchaeccYbe. 

 Co/lencyte 



DertnalceU — 



Choanoo/ie -I 

 (Oslhrceli) 



Porocyte s^. 



Ovum '4^~ 



Fig. 54. — Histology of wall of a simple sponge in longitudinal section. (Redrawn 

 and modified from Lankester, Treatise on Zoology, published by The Macmillan 

 Company, after Minchin.) 



tered, flat, epithelial cells. Each choanocyte has at its free margin 

 a funnel or collar opening to the central cavity and a flagellum or 

 whip extending from the funnel. The flagella agitate the water and 

 drive the suspended food particles into the funnellike mouths of 

 the collar cell where a food vacuole is formed in the cell in amoeboid 

 fashion. Of course the spicules appear in a histological section. 

 The entire arrangement is quite similar to a large colony of semi- 

 independent cells which do not function as integral parts of a tis- 

 sue as do the cells of higher animals. It has been found that indi- 



