1 64 



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 1897. 



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 



FIG. 74. Three simple metazoans of these sienS Sponges of 



isolated structural types. 1 * r 



A. a scruff back, ChoUmctus; B. Hydra, bearing a Whatever torm Or 



ud; C. a tardigrade. Macrobiotus. 



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, 

 like daffodils. 



