SECTION I. EFFECTOR SYSTEMS 



CHAPTER II 

 SPONGES 



SPONGES are such inert organisms that their member- 

 ship in the animal series was for a long time unsuspected. 

 Their methods of development, however, settled this 

 question beyond dispute. In form they are either single, 

 more or less goblet-shaped animals (Fig. 4) or somewhat 

 amorphous colonies whose chief activity is exhibited in 

 the very considerable currents that they produce in the 

 surrounding water. These currents enter the substance 

 of the sponge through innumerable pores scattered over 

 the surface of its body, pass through its more solid parts 

 by a system of canals that converge on a central cavity, 

 the cloaca, from which they emerge by a large opening, 

 the osculum. This opening is situated at the apex of the 

 individual sponge or, in the case of a colony, each osculum 

 is ordinarily on a somewhat conical elevation rising above 

 the general surface of the colony. 



The currents that emerge from sponges are often so 

 considerable as to deform the surface of the water above 

 them much as is done by a vigorous spring. They are 

 produced by the action of the choanocytes or flagellated 

 cells that line the canals in the body of the sponge or that 

 invest more specialized chambers interpolated on the 

 course of these canals. So far as can be judged by the 

 examination of the inner parts of sponges, the choano- 

 cytes are incessantly active, and the obliteration and re- 

 vival of currents as seen in many of these animals are 



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