108 



ZOOLOGY 



FIG. 81. Sycon grelatinosum Transverse section 

 through the wall of a cylinder (parallel with the 

 course of the canals), showing one incurrent (1C), 

 and one radial (R) canal throughout their length ; 

 xp. triradiate spicules ; sp'. oxeote spicules of dermal 

 cortex (rfc.) ; sp". tetraradiate spicules of gastral 

 cortex ((]c.) ; ec. ectoderm ; en. layer of flattened cells 

 lining the paragastric cavity ; pm. pore-membrane ; 

 pp. prosopyles ; ap. apopylc ; di. diaphragm ; exc. 

 excurrent passage ; P.G. paragastric cavity ; em. 

 early embryo ; em', late embryo. The arrows in- 

 dicate the course of the water through the sponge. 



SECT- 



the skeleton of the 

 Sycon. 



The arrangement of the 

 spicules, their relation to 

 the protoplasmic parts, 

 and the structure of the 

 latter, have to be studied 

 in thin sections of hard- 

 ened specimens (Figs. 80 

 and 81). An examination 

 of such sections leads to 

 the following results. 



Microscopic struc- 

 ture. Covering the outer 

 surface of the sponge is 

 a single layer of cells the 

 dermal layer or ectoderm l 

 (Fig. 81, ec) through 

 which project regularly- 

 arranged groups of needle- 

 like and spear-like spicules 

 (sp'), forming the pattern 

 of polygonal elevations on 

 the outer surface. The 

 cells of the ectoderm 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 ectoderm, thin 

 flattened scales. Running 

 radially through the thick 

 wall of the cylinders arc a 

 large number of regularly- 

 arranged straight passages. 

 Of these there are two sets, 

 those of the one set the 

 incurrent canals (Figs. 80 



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. 



