THE CALCAREOUS SPONGES 3511 



On making a section or teasing a fragment of a calcareous sponge in the spring, 

 the minute embryos will often be seen. When the embryo leaves the parent sponge, 

 it consists of an extremely minute oval cyst or vesicle (v) with a small central 

 cavity, and is formed of two kinds of cells. The anterior half, or the part in front 

 when the embryo is swimming, is composed of a number of long prismatic cells, 

 each carrying a vibratile flagellum. The hinder end is composed of a smaller num- 

 ber of large rounded granular cells without flagella. 



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



DEVELOPMENT OF Sycon raphanus (all figures enlarged). 



zone (). 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 

 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. Calca- 

 reous 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. 



