PORIFERA 



31 



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Fig. 10.— Diagram of the development of the canal system in various sponges. 

 The ectoderm is indicated by a continuous, and the entoderm by a broken, line. 

 The collar epithelium of the entoderm is e.xpressed by perpendicular striations. 

 A, cross section through a part of the wall of an ascon ; B, cross section through 

 the wall of a sycon; O, cross section through the wall of Leucilla connexiva (after 

 Polejaeff) ; D, vertical section through Oscarella ; a, spaces of the inhalent 

 canal system; b, spaces of the exhalent canal system ; os, osculum. 



