SPONGES 



21 



without doubt, many-celled animals. If a living sponge is cut 

 into pieces, each piece will grow and form a complete sponge. 



That the sponge is not a colony of one-celled animals, each like 

 an amoeba, but is a many-celled animal, will be realized by exam- 

 ining Fig. 32, which shows a bit of sponge highly magnified. A 

 sponge may be conceived as having developed from a one-celled 

 animal as follows : Sev- 

 eral one-celled animals 

 happened to live side by 

 side ; each possessed a 

 thread-like flagellum (E, 

 Fig. 32) or whip-lash for 

 striking the water. By 

 lashing the water, they 

 caused a stronger cur- 

 rent (Fig. 25) than pro- 

 tozoans living singly 

 could cause. Thus they 

 obtained more food and 

 multiplied more rapidly 

 than those living alone. 

 The habit of working 

 together left its impress 

 on the cells and was trans- 

 mitted by inheritance. 



Cell joined to cell 

 formed a ring ; ring 

 joined to ring formed a tube which was still more effective than 

 a ring in lashing the water into a current and bringing fresh food 

 (particles of dead plants and animals) and oxygen. 



Few animals eat sponges; possibly because spicules, or fibres, 

 are found throughout the flesh, or because the taste and the odour are 

 unpleasant enough to protect them. Small animals sometimes 

 crawl into sponges to hide. One sponge grows upon shells in- 

 habited by hermit crabs. Moving of the shell from place to place 

 is an advantage to the sponge, while the sponge conceals and thus 

 protects the crab. 



Special Report : Sponge" Fisheries." (Localities; how sponges 

 are taken, cleaned, dried, shipped, and sold.) 



FlG. 32. — Microscopic plan of ciliated chamber. 

 Each cell lining the chamber has a nucleus, 

 a whip-lash, and a collar around base of 

 whip-lash. Question : State two uses of 

 whip-lash. 



