Chap. 22 SPONGES — a side line of evolution 461 



the eyed Finger Sponge {Chalina oculata) of the Atlantic coast and Leu- 

 cosolenia eleanor of the Pacific coast. Buds broken off and carried by currents 

 established themselves in protected crannies and other places, such as wharf 

 pilings and the backs of cjrabs. 



Fresh-water sponges are mainly annual growths that die out in autumn ex- 

 cept for the gemmules that can resist both drought and cold. These winter 

 over in safety and germinate into young sponges in the spring (Figs. 22.7, 

 22.8). A gemmule is a ball of foodfilled ameboid cells and mesenchyme 

 enclosed within a capsule. The outer wall is pierced by a minute outlet through 

 which the growing sponge spreads forth. In autumn, the flourishing summer 

 colonies of Spongilla are reduced to thousands of spicules sticking to the rock 

 with many gemmules appearing like fig seeds packed among them. 



All animals, especially invertebrates, have some power to replace lost or 

 injured parts. With their relatively simple organization, sponges have a great 

 capacity for these processes of regeneration even to the extent of a complete 

 rearrangement of their parts after they are separated. When certain sponges 

 are pushed through silk bolting cloth, their cells are nearly all separated from 

 one another. If the redbeard sponge, Microciona, is thus treated and its cells 

 allowed to fall into a large flat dish of sea water, they will spread and the 

 solution soon resembles tomato soup. The amebocytes immediately begin 

 random movements and certain of them become centers about which special 

 food-carrying amebocytes congregate (Fig. 22.9). Collared cells that have 

 been injured regrow their collars and take their proper places as living cells of 



Fig. 22.7. Spongilla and other fresh-water sponges frequently overwinter as 

 gemmules which resist cold and drying. Held among the spicules of the summer 

 colony they look like fig seeds caught in the meshes of torn lace. 



