CHAR A C TER IS TICS. 
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 
A, Diagram of canal system. D, 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. Plahina, 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,” 
