Ill PHYLUM AND CLASS PORIPERA 109 



and 81 IG) — narrower, and lined by ectoderm similar to the ectoderm 

 of the surface ; those of the other set^ — the radial or flaciellate canals 

 (iJ) — rather wider, octagonal in cross-section, and lined by endoderm 

 continuous with the lining of the paragastric cavity. The incurrent 

 canals end blindly at their inner extremities — not reaching the 

 paragastric cavity ; externally each becomes somewhat dilated, 

 and the dilatations of neighbouring canals often communicate. 

 These dilated parts are closed externally by a thin membrane — 

 the pore-membrane (Fig. 81 pm, and Fig. 82), perforated by three 

 or four small openings (Fig. 82, p) — the ostia already referred to. 

 The flagellate canals are blind at tlieir 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 exctorrent canal (Fig. 81 exc). Incurrent 



N. 



Fio. 82.— Sycon gelatinosnm. Svir- Fm. 83.— Sycon gelatinosum. 



face view of a pore-niembr.ane highly An apopyle surrounded by its dia- 



maguified : p. ostium ; K. position of pliragm ; m, contractile cells, 

 the outer end of a radial canal. 



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 proso- 

 pyle is a perforation in a single cell termed a piorocyte. 



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

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

 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. 77) ; 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 



