Ill 



PHYLUM AND CLASS PORIFERA 



100 



ectoderm 1 (Fig. 82, ec) 

 through which project 

 regularly-arranged groups 

 of needle-like and spear- 

 like spicules (sp r ), forming 

 the pattern of polygonal 

 elevations on the outer 

 surface. The cells of 

 the ectoderm (pinacocytes) 

 are in the form of thin 

 scales, which are closely 

 cemented together by their 

 edges. The paragastric 

 cavities are lined by a 

 layer of cells (en) which 

 are, like those of the ecto- 

 derm, thin flattened scales. 

 Running radially through 

 the thick wall of the cylin- 

 ders are a large number of 

 regularly-arranged straight 

 passages. Of these there 

 are two sets, those of the 

 one set the incurrent 

 canals (Figs. 81 and 82, 1C) 

 narrower, and lined by 

 ectoderm similar to the 

 ectoderm of the surface ; 

 those of the other set the 

 radial ox flagellate canals (R) 

 rather wider, octagonal 

 in cross-section, and lined 

 by endoderm continuous 

 with the lining of the 

 paragastric cavity. The 

 incurrent canals end blind- 

 ly at their inner extremi- 

 ties not reaching the 

 paragastric cavity ; ex- 

 ternally each becomes 

 somewhat dilated, and 



1 The terms ectoderm and 

 endoderm are here used as con- 

 venient terms for the outer and 

 inner layers of the Sponge, 

 though, as will appear later, 

 these layers differ completely 

 in their mode of formation 

 from the layers so named in the 

 higher phyla. 



Fia.82. Sycon gelatinosum. Transverse section 

 through the wall of a cylinder (parallel with the 

 course of the canals), showing one incurrent (/C), 

 and one radial (R) canal throughout their length ; 

 sp. triradiate spicules ; sp'. oxeote spicules of 

 dermal cortex (dc.) ; sp". tetraradiate spicules of 

 gastral cortex (go.) ; ec. ectoderm ; en. layer of 

 flattened cells lining the paragastric cavity ; pm. 

 pore -membrane ; pp. prosopyles ; ap. apopyle ; 

 di. diaphragm ; exc. excurrent passage ; P.O. para- 

 gastric cavity ; em. early embryo ; em', late em- 

 bryo. The arrows indicate the course of the water 

 through the sponge. 



