100 ZOOLOGY SECT. 



and 727(7) narrower, and lined by ectoderm similar to the ectoderm 

 of the surface ; those of the other set the radial or flagellate canals 

 (R) 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. 72, pin, and Fig. 73), perforated by three 

 or four small openings (Fig. 73, p) the inhalant pores 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 para- 



i- 



FIG. 73. Sycon gelatinosum. Sur- FIG. 74. Sycon gelatinosum. 



face view of a pore membrane highly An apopyle surrounded by its di;v 



magnified ; p. inhalant pore ; R, posi- phragm ; m. contractile cells, 

 tioii of the outer end of a radial canal. 



gastric cavity by a short wide passage the excurrent canal 

 (Fig. 72 cxc). 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 

 prosopylcs (pp), uniting the cavities of adjacent incurrent and 

 flagellate canals. 



The ectoderm lining the incurrent canals is of the same character 

 as the syncytium 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. 72); 

 it has its nucleus, one or more contractile (?) 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 



