SPONGES 



193 



Here the skeleton 

 Some of the rarest 



Venus's flower basket: a 

 sponge with a glassy skeleton. 



ectoderm; the inner layer, developed from the inner layer of the 

 gastrula, the endoderm. A middle structureless layer, called the 

 mesoglea, is also found. In higher animals 

 this layer (called mesoderm) gives rise to 

 muscles and parts of other internal struc- 

 tures. 



Other Sponges. — Sponges may be placed, 

 according to the kind of skeleton they possess, 

 in the following groups: 



(1) The limy sponges, in which the skeleton 

 is composed of spicules of carbonate of lime. 

 Grantia is an example. 



(2) The glassy sponges. 

 is made of silica or glass. 



and most beautiful of all sponges belong in this 

 class. The Venus's flower basket is the best 

 known. 



(3) The horny fiber sponges. These, the 

 sponges of commerce, have the skeleton com- 

 posed of tough fibers of material somewhat like 

 that of cow's horn. This fiber is elastic and has the power to absorb water. 

 In a hving state, the horny fiber sponge is a dark-colored fleshy mass, 



usually found attached to rocks. 

 The warm waters of the Mediter- 

 ranean Sea and the West Indies 

 furnish most of our sponges. The 

 sponges are pulled up from their 

 resting place on the bottom, either 

 by means of long-handled rakes 

 operated by men in boats, or are 

 secured by divers. They are then 

 spread out on the shore in the sun, 

 and the living tissues allowed to 

 decay; then after treatment con- 

 sisting of beating, trimming, and 

 bleaching, the bath sponge is ready 

 for the market. 



Relation to Environment. — 

 Sponges are found in both fresh 

 . , „, r D .1, • ^,,+ and salt water.^ They are found 



A homy fiber sponge; I.P., the mcurrent 



pores; 0, osculum. Notice that this sponge ^ Only one species of fresh water 



is made up of apparently several individuals. sponge, Spongilla, is known in this 

 One fourth natural size. country. 



HUNTER^S BIOL. — 13 



