CHAP A CTERISTICS. 



533 



be so close to one another that the spaces between and outside of them form 

 channels, which are incipient in-current canals, the spaces in the inside or lumen 

 of the folds forming the out-current canal-system. 



The common ciliated sycon, a calcareous sponge found on seaweeds round the 

 British coast, forms a white sac about an inch in height, and with a crown of 

 glassy bristles round the orifice. The vertical cavity of the sac is surrounded by a 

 wall of closely packed horizontal tubes, opening at their inner ends into the central 

 cavity, but externally ending blindly. The central cavity of the sac is lined with 

 flat-cells, and the radial tubes with collar-cells; and the walls of the tubes are 

 perforated with small pores. Here the spaces between and outside the densely 

 packed tubes are the in-current canals. In an equally common British sponge, 

 Grantia, which forms small flat white bags, a rudimentary cortex covers the 

 outer ends of the tubes. In Grantiopsis the cortex becomes quite thick. In 

 more complex stages the radial tubes branch ; and, finally, the collar cells clothe 

 only the ends of branched tubes, thus giving rise to more or less spherical 

 flagellated chambers. As the radial tubes become more branched, and the 



TOILET-SPONGE. 



A, Diagram of canal system. B, Section showing a, pores ; b, canals ; c, flagellated chambers ; d, skeleton-fibres; 

 d', main fibre ; e, embryo eggs. C, Flagellated whip-chambers. (Highly magnified.) After F. E. Schulze. 



mesoderm thicker, so the passages or in-current canals from the outside of the 

 sponge to the outside of the radial tubes become more complicated. Common 

 siliceous sponges develop in a different manner from the above-described calcareous 

 ones, namely, from a hollow conical sac open at the top, and with a flat base ; the 

 spherical flagellated chambers at a very early stage forming a mammillated layer 

 in the walls. Plakina, one of the simplest siliceous sponges, encrusts stones with 

 a fleshy crust, consisting of a sac with a flat base attached to the stone, and with the 

 rest of the walls forming simple folds. The spaces between and outside the folds 

 form the in-current, and those in the lumen of the folds the out-current channels. 

 Each of the flagellated chambers in the walls of the folds communicates with the 

 in-current spaces through several pores, and opens into the out-current spaces by 

 one large pore, the currents of water passing out by the central oscule. 



The fine toilet-sponge possesses a more developed canal-system. The in-current 

 and out-current parts of the water-bearing system are more definitely " canalised," 



