PHYLUM AND CLASS POR1FERA 



99 



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. 71 

 and 72). 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 

 ectoderm (Fig. 72, ec) 

 through which project 

 regularly-arranged groups 

 of needle-like and spear- 

 like spicules (sp), form- 

 ing the pattern of poly- 

 gonal elevations on the 

 outer surface. The cells 

 of the ectoderm are in the 

 form of thin scales, which 

 are closely cemented to- 

 gether by their edges to 

 form a syncytium, or mem- 

 brane consisting of cells 

 so intimately united that 

 their boundaries are not 

 readily distinguishable. 

 The paragastric cavities 

 are lined by a layer of 

 cells (en) which are like 

 those of ' the ectoderm, 

 but are somewhat thicker 

 and more granular : this is 

 the endoderm of the para- 

 gastric cavity. Running 

 radially through the thick 

 wall of the cylinders are a 

 large number of regularly- 

 arranged straight passages. 

 Of these there are two sets, 

 those of the one set the 

 incurrcnt canals (Figs. 71 



J 



FIG. 72. Sycon gelatinosum. Transverse section 

 through the wall of a cylinder (parallel with the 

 course of the canals), showing one incurrent canal 

 (1C), and one radial (R) throughout their length ; 

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

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

 cortex ((/P.) ; ec. ectoderm ; en. endoderm ; pm. pore 

 membrane ; pp. prosopyles ; ap. apopyle ; di. dia- 

 phragm ; exc. ^excurrent passage ; P.G. paragastric 

 cavity ; em. early embryo ; em', late embryo. The 

 arrows indicate the .course of the water through 

 the sponge. 



H 2 



