sect, m PHYLUM PORIFERA 81 



flagellate canals (j?) — 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 

 dilations of neighbouring canals often communicate. These 

 dilated parts are closed externally by a thin membrane — 

 the pore membrane, perforated by three or four openings — 

 the inhalant pores already referred to. The flagellate canals 

 are blind at their outer ends, which lie at a little distance 

 below the surface ; internally, each communicates with the 

 paragastric cavity by a short, wide passage, the excurrent 

 canal (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 com- 

 munication — the prosopyles (pp) — uniting the cavities 

 of adjacent incurrent flagellate canals. 



The ectoderm lining of the incurrent canals is of the 

 same character as the ectoderm 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, as they are termed, is not 

 unlike one of the choanoflagellate Protozoa (p. 38) ; it has 

 its 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 

 examined under a tolerably high power of the microscope, 

 groups of these collared cells will be detected here and 



