110 ZOOLOGY . SECT. 



the dilatations of neighbouring canals often communicate. 

 These dilated parts are closed externally by a thin membrane 

 the pore-membrane (Fig. 82, pm, and Fig. 83), perforated by three 

 or four small openings (Fig. 83, p) the ostia already referred 

 to. The flagellate canals are blind at their outer ends, which lie at a 

 little distance below the surface opposite the polygonal projections 

 referred to above as forming a pattern on the outer surface ; 

 internally, each communicates with the paragastric cavity by a 

 short, wide passage the excurrent canal (Fig. 82, exc.) Incurrent 

 and flagellate canals run side by side, separated by a thin layer of 

 sponge substance except at certain points, where there exist small 

 apertures of communication the prosopyles (pp) uniting the 

 cavities of adjacent incurrent and flagellate canals. Each prosopyle 

 is a perforation in a single cell termed a porocyte. 



The ectoderm lining the incurrent canals is of the same char- 

 acter as that of the outer surface. The endoderm of the 



FIG. 83. Sycon gelatinosum. Sur- FIG. 84. Sycon gelatinosum. 



face view of a pore-membrane highly An apopyle surrounded by its dia- 



magnified ; p. ostium ; R. position of phragm ; m. contractile cells, 

 the outer end of a radial canal. 



flagellate canals, on the other hand, is totally different from that 

 which lines the paragastric cavity. It consists of cells of columnar 

 shape ranged closely together so as to form a continuous layer. 

 Each of these flagellate endoderm cells, or collared cells, or choano- 

 cytes, as they are termed, is not unlike one of the Choanoflagellate 

 Protozoa (p. 79) ; it has a nucleus, one or more vacuoles, and, at 

 the inner end, a single, long, whip-like flagellum, surrounded at its 

 base by a delicate, transparent, collar-like upgrowth, similar to 

 that which has already been described as. occurring in the 

 Choanoflagellata. If a portion of a living specimen of the sponge 

 is teased out in sea-water, and the broken fragments are examined 

 under a tolerably high power of the microscope, groups of these 

 collared cells will be detected here and there, and in many places 

 the movement of the flagella will be readily observed. The 

 flagellum is flexible but with a certain degree of stiffness, 



