PORIFERA 



51 



but little developed, the sponge reacts to stimuli. G. H. Parker has 

 shown that the oscula (of Stylotelld) were closed in quiet sea water 

 and on exposure to the air and to ether. The ostia (pores) opened 

 in sea water currents and in fresh water and atropine, and closed in 

 weak ether and cocaine. 



Habits. — Sponges furnish shelter for small organisms. They are 

 inactive (sessile) and only open and close their openings. They are 

 not eaten by fishes or even Arthropoda. Their strong odor and 





r' 



Fig. 



:o. Clam shell infested with boring sponge. (From A. G. Mayer, Seashore Life. 

 Courtesy of New York Zool. Soc.) 



taste are important aids to the spicules in keeping enemies away. 

 Micro-organisms that find their way through the pores are taken in 

 as food by the phagocytic cells of the cloaca and radial canals. 

 Sponges are not true parasites, but the boring sponge, C/iona, 

 perforates the shell of oysters and other similar forms, seeking ^ro/^f- 

 tion instead of food. (Figure 20.) 



Enemies of the sponges are bacteria, plant parasites and a few 

 fish, which attack them when they are young. 



Associations. — Certain species of crabs (Dromia) are masked by 

 sponges living as commensals, which profit by securing more oxygen. 



