GENERAL ZOOLOGY 



Osculum 



1 1 17 



Ostia 



Fig. 9.10. Comparison of various types of sponge structure; diagrammatic. The dark areas 

 represent choanocyte layers; arrows indicate direction of water currents. .4, asconoid type, 

 with spongocoel completely lined by choanocytes. B, simple syconoid type, with choanocytes 

 in excurrent canals only. C, more complex syconoid type, with thickened walls and narrowed 

 openings into incurrent canals. D, primitive leuconoid type, with excurrent canals in groups 

 opening into branches of the spongocoel. E, more advanced leuconoid type; the choanocytes 

 are restricted to small, spherical flagellated chambers (Jc) which are considerably removed 

 from both the outer surface and the spongocoel. (Redrawn with modifications from E. A. 

 Minchin, in l.ankester's Treatise on ^oolooy, 1900.) 



Few sponges are symmetrical like Leiicosglema and Scypha, although the 

 radial symmetry of these genera and of the olynthus appears to be the primi- 

 tive state from which all sponges have been derived in the evolution of the 

 phylum. Moreover, there may be great differences in shape among the in- 

 dividuals of the species, for the growth of a sponge is much influenced by con- 

 ditions in the immediate environment. 



Metabolism. The food of sponges consists of microorganisms and particles 

 of organic detritus which enter the canal systems with the inflowing water 



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