3508 



THE SPONGES 



framework for the pores. By eliminating the spicular skeleton, and by supposing 

 the tube or vase to be more globular, we obtain the ' ' olynthus-form ' ' which has 

 been regarded as the hypothetical ancestor of all sponges. A canal system arises 

 when the walls grow thick or form folds or give off pouches or tubes. The folds 

 or pouches may 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 



oscul< i 



TOILET SPONGE. 



A. Diagram of canal system. . 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. 



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 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, in- 

 crusts 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 



