SPONGES 



131 



and each group will grow into a sponge. This illustrates the semi- 

 independent character of the cells. 



154. Metabolism. — Metabolism is carried on practically in the 

 same manner as it is in Protozoa. That there are, however, different 

 enzymes acting on proteins, carbohydrates, and fats seems to be generally 

 accepted. 



The food of sponges consists of minute plants and animals and also 

 small particles of organic matter which are drawn into the ostia and 

 through the canals by currents produced by the movement of the flagella 

 of the collar cells. As this current 

 sweeps these objects past the collar 

 cells they are seized upon by the 

 cells and ingested by means of pseudo- 

 podia. The current of water pro- 

 ceeds onward into the gastral cavity 

 and out of the body through the 

 osculum. The food which has been 

 taken by the collar cells is digested 

 in food vacuoles in the same manner 

 as it would be digested by proto- 

 zoans. Further steps in metabolism \j 

 also occur like those in protozoans. 

 Each cell excretes and respires for 



a 



Fig. 52. — Types of spicules. {From 

 Sollas, "Cambridge Natural History," 

 by the courtesy of The Macmillan Com- 

 itself. The cells which are not collar pany.) a and b, monaxon; c, triradiate; 



cells receive their food 



d, tetraxon; e, triaxon;/, polyaxon. 



receive tneir lood more or 

 less directly from the latter by absorption from cell to cell, aided by the 

 ameboid wandering cells, which serve to carry both food and waste 

 matter about the body. 



155. Behavior. — Little is known of behavior in sponges generally. 

 The larvae are ciliated and swim about, but the adults are attached 

 and never move from their position. Some sponges possess fiber-like 

 cells around the ostia and oscula which are capable of slowly contracting 

 and closing these openings or of relaxing and permitting them to open. 

 The opening and closing are so gradual, however, that they do not attract 

 notice unless particular attention is given to them. These openings tend 

 to open when the water is in motion but close when the water becomes 

 quiet; they also open in fresh water and in weak solutions of atropin and 

 close on exposure to the air or on injury to the animal or when the animal 

 is subject to the experimental action of weak solutions of ether and 

 cocaine. These fiber-like cells, since they have the function of both 

 receiving stimuli and contracting in response to them, are termed neuro- 

 muscuLar cells. Because groups of these cells surround openings which 

 are closed by their contraction, the group is termed a sphincter. (This 

 term is also applied to all muscles closing openings in the bodies of higher 



