THE METAZOA DIVISION OF LABOR 



207 



FORMS OF SIMPLE METAZOANS. 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. Here the skeleton 

 is made of silica or glass. Some of the rarest 

 and most beautiful of all sponges belong in 

 this class. The Venus' s flower basket is an 

 example. 



(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 

 living 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 consisting of beating, bleaching, and trimming, the bath 



sponge is ready for the mar- 

 ket. 



CCELENTERATES 



The hydra and its salt- 

 water allies, the jellyfish, 

 hydroids, and corals, belong 

 to a group of animals known 

 as the Ccelenterata. The word 

 " coelenterate " (cadom = body 

 cavity, enteron=food tube) 

 explains the structure of the 

 group. They are animals 

 which have a common body 

 cavity and food tube, the 

 animal in its simplest form 

 being little more than a 

 bag. 



Venus's flower basket; a 

 sponge with a glassy skele- 

 ton. 



Medusa (Gonionemus murbachii), showing ten- 

 tacles, mouth, digestive canals, and reproduc- 

 tive bodies. Photographed from the model 

 at the American Museum of Natural History. 



