90 ELEMENTARY ZOOLOGY 



viduals, a colony of sponges is formed which has the 

 general appearance of a branching plant. In other 

 species the new sponge formed by the development and 

 growth of a bud falls off and becomes a distinct separate 

 individual. 



In the sexual mode of reproduction, male or sperm- 

 cells and female or egg-cells are developed in the same 

 individual. The sperm-cells are motile and swim about 

 in the cavities and canals of the sponge-body until they 

 find egg-cells, which they fertilize. The fertilized eggs 

 begin to develop and pass through their first stages in the 

 sponge-body. Finally the embryo sponge, which is 

 usually a tiny oval or egg-shaped mass of cells, escapes 

 from the body of the parent into the water. The young 

 sponge has some of its outer cells provided with cilia, 

 and by means of these it swims about. After a while 

 it comes to rest on the ocean-floor or on some rock 

 or shell, attaches itself, and begins to take on the form 

 and character of the parent. It leads hereafter a fixed 

 sedentary life. 



The sponges of commerce. The sponge-skeletons 

 which are the ' ' sponges ' ' that we use all belong to a few 

 species, not more than half a dozen. Most of the com- 

 mercial sponges come from the Mediterranean Sea, though 

 some come from the Bahama Islands, some from the Red 

 Sea, and a few from the coasts of Greece, Asia Minor, 

 and Africa. The commercial sponges do not live in very 

 deep water; they are usually found not deeper than 200 

 feet. The living sponges are collected by divers, or are 

 dragged up by men in boats using long-poled hooks, or 

 dredges. ' When secured they are exposed to the air 

 for a limited time, either in the boats or on shore, and 

 then thrown in heaps into the water again in pens or 

 tanks built for the purpose. Decay thus takes place with 

 great rapidity, and when fully decayed they are fished up 



