140 



THE RISE OF 

 excurreni 



Ascon 



*f'^ 



incurrent 



^ ■ %^ Sycon 



Fig. 8-5. Various types of canal systems among sponges. 



from its enemies. Sponges can become a 

 hindrance in oyster beds where they cover 

 the oysters and compete for the food 

 supply. 



ANIMAL LIFE 



Sponges have canal systems which vary 

 in complexity (Fig. 8-5). The simplest, the 

 Ascon type found in Leucosolenia, is 

 merely a thin-walled sac. The first step in 

 increasing complexity occurs in the Sycon 

 type, which has two kinds of canals, incur- 

 rent and radial, the latter being lined with 

 choanocytes. The most complicated of all is 

 the Leucon type, which possesses a vast 

 array of multiple tubes and chambers, with 

 choanocytes lining only certain restricted 

 chambers. Leucon sponges may reach more 

 than a meter in diameter. 



Sponges have been used by man for 

 cleaning purposes from earliest times. The 

 part used in the bath sponge is the skeleton, 

 composed largely of spongin, a protein. It 

 has many fine fibers which, through capil- 

 larity, have remarkable water-holding prop- 

 erties. Sponges for commercial use are 

 found most frequently in the warm seas 

 such as the Mediterranean, the Gulf of 

 Mexico, and the Gulf of Florida. In prepa- 

 ration for market, sponges are pried loose 

 from rocks by divers or dredges, and the 

 living portion is allowed to dry and decay. 

 They 'are then beaten, washed, and finally 

 bleached. Needless to say, the bath sponge 

 has considerable commercial value. In re- 

 cent years it has even become necessary to 

 cultivate it in order to prevent it from be- 

 comins extinct as the result of the constant 

 ravages of man. Two million pounds of 

 sponges are taken annually, even though 

 the sponge must compete with its rubber 

 and plastic imitations. 



In review, we may think of the sponges 

 as "blind alley" animals that came to their 

 present state millions of years ago and have 

 "stood still," failing to evolve any higher on 

 the animal scale. They are sufficiently well 

 adapted to their environment to carry on in 

 their "primitive way and they probably will 

 continue for millions of years to come. 

 Since they are not on the direct line of 

 ascent to higher forms, we must leave them 

 in their isolated position without further 

 reference to them and pass on to the next 



