536 



SPONGES. 



prismatic cells, each carrying a vibratile flagellum. The hinder end is composed of 

 a smaller number of large rounded granular cells without flagella. 



After the larva has swum about for a time, it becomes broader in the 

 equatorial zone (6). The fore-half broadens out more and more till it forms a 

 flat lid on the hinder hemisphere. Finally, the small cells lose their cilia and 

 become completely invaginated into the interior of the large-celled hemisphere, 

 which now resembles a cup with a double wall (c). 



There are thirteen British species of Calcarea, and over two hundred from all 



w/)\wm 



development OP Sycon raphanus (all figures enlarged). 



parts of the world. They are almost confined to shallow water, the greatest depth 

 from which they have been obtained being four hundred and fifty fathoms. 

 Calcareous sponges prefer shade and avoid light, and are chiefly found in caves, 

 under stones, in shells, or in the shade of dense thickets of seaweed. 



Six-Rayed, or Glass-Sponges, — Class Hexactinellida. 



In the sponges of this group the skeleton is built of spicules with three axes 

 and six rays, intersecting one another at right angles through a common centre. 

 A second characteristic consists in the comparatively simple arrangement and large 



