EVOLUTION OF NERVOUS SYSTEM 91 



production and control of this current is one of its essential 

 activities. 



Many sponges, like the ordinary bath sponge, are colonial 

 animals and consist of a number of sponge individuals more or 

 less fused together. In the bath sponge, which in its commer- 

 cial form is represented merely by its horny skeleton, the num- 

 ber of individuals can be judged by the number of very large 

 openings that penetrate from the outside to its interior. These 

 are usually four, five, or more in number and represent the 

 outlets for the water currents in the living sponge. In quite 

 a number of the colonial sponges the component individuals 

 are much more separated than in the bath sponge and rise 

 from a common base as so many separate fingers. Such 

 fingered sponges are very convenient for study and are com- 

 monly of such a size as to admit of easy experimental treat- 

 ment. 



Sponges are for the most part extremely inert and aside 

 from a slight contraction and consequent bending of the body 

 as a whole they show very few activities except the opening 

 and closing of their pores and other apertures. In this way 

 sponges control their water currents and they accomplish 

 this partly by the formation of protoplasmic membranes by 

 which the pores are closed and partly by the action of rings of 

 simple muscle cells that surround the pores, especially the out- 

 let openings. 



The control of a given outlet cannot be accomplished from 

 an adjacent finger nor can it be brought about by stimulating 

 the finger at the end of which the outlet lies except when the 

 stimulus is directed to the immediate neighborhood of the 

 outlet itself. In fact there is not only no experimental evi- 

 dence to show that one finger is connected nervously with 

 another, but there is no reason to suppose that parts of the 

 same finger are thus connected. In other words, nervous 



