164 



Aquatic Organisms 



quent detachment, they become numerous when 

 plancton abounds. Kofoid ('08) found a maximum 

 number of 5335 hydras per cubic meter of water in 

 Quiver Lake during a vernal plancton pulse in 1S97. 



Fresh-water sponges grow abundantly in the margins 

 of lakes and pools and in clear, slow-flowing streams. 

 They are always sessile upon some solid support. In 

 sunlight they are green, in the shade they grow pale. 

 The species that branch out in slender finger-like pro- 

 cesses are most suggestive of plants in both form and 



color, but even the slen- 

 derest sponge is more 

 massive than any plant 

 body; and when one 

 looks closely at the 

 surface he sees it rough- 

 ened all over with the 

 points of innumerable 

 spicules, and sees open 

 osteoles at the tips. By 

 these signs sponges of 

 whatever form or color 

 are easily recognized. 

 The commonest sponges are low encrusting species 

 that grow outspread over the surfaces of logs and 

 timbers. When, in early summer, one overturns a 

 floating log that has been long undisturbed he may find 

 it dotted with young sponges, growing as little yellow. 

 circular, fleshy discs, bristling with spicules, and each 

 with a large central osteole. Later they grow irregular 

 in outline, and thicker in mass. Toward the end of 

 their growing season they develop statoblasts or 

 gemmules (winter-buds) next to the substratum (see 

 fig. 164 on p. 264), and then they die and disintegrate. 

 So our fresh-water sponges are creatures of summer, 

 * 4 ils. 



Fig. 74. Three simple metazoans 



isolated structural types. 

 .1. a scruff back, Chatonctw;; B. Hydra, bearing a 

 bud; C, a tardigrade. Macrobiotus. 



