SECT, in PHYLUM PORIFERA 81 



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, 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 (ex. c.). 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 and 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 (R\ on the other hand, is 

 totally different from that which lines the paragastric cavity. 

 It consists of cells of columnar shape, arranged closely to- 

 gether 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 

 there, and in many places the movements of the flagella 

 Man. Zool. 



