102 COLLEGE ZOOLOGY 



(2) Physiology. — Metabolism. — The metabolic processes 

 in all sponges are essentially similar to those of Grantia (p. 96). 

 The current created by the beating of the flagella of the choano- 

 cytes brings organic food particles and fresh water into the canals. 

 Most of the food particles are engulfed by the choanocytes and 

 digested within the cells, as in Protozoa. The processes of ex- 

 cretion and respiration are carried on by the cells of the body- 

 wall. There is, on the whole, not much difference between the 

 metabolic activities of sponges and those of Protozoa. 



Behavior. — Very little is known about the behavior of 

 sponges. The larvae, as stated before, are ciliated and swim 

 through the water, but the adults are all attached to the sea- 

 bottom, to rocks, or to piles, etc. Parker has shown that Stylo- 

 tella heliophila, of the order Monaxonida, responds in a prim- 

 itive way to certain stimuli. Among the reacting elements are 

 fiber-like ceils, myocytes, arranged about the osculum, and con- 

 tractile cells lining certain internal cavities. The choanocytes 

 are able to extend and contract their collars and to beat the water 

 with their flagella. No nervous elements have been discovered. 



The reactions of Stylotella may be briefly stated as follows: — 



The oscula close in quiet sea- water, on exposure to air, on in- 

 jury to neighboring parts, and in weak solutions of ether and 

 cocaine; they open in currents of sea-water, in fresh water, and 

 in weak solutions of atropine. 



The ostia close on injury to neighboring parts and in weak 

 solutions of ether and cocaine; they open in dilute sea- water, 

 and in weak solutions of atropine. The choanocyte currents 

 cease in dilute sea- water, at high temperatures, ahd in weak solu- 

 tions of ether and chloroform. There is very little, if any, trans- 

 mission of stimuli, and the reactive organs respond only to 

 direct stimulation. 



Investigators look to the lowly organized, many-celled animals 

 for the key to the origin of the nervous system, and the condition 

 in sponges seems to show that muscles, " as represented by the 

 sphincters of sponges, were the first of the neuromuscular organs 



