120 Aquatic Organisms 



is in a median longitudinal plate, which can rotate in 

 the cell: it tiirns its thin edge upward to the sun, but 

 lies broadside exposed to weak light. Spirogyra is 

 the most abtmdant, especially in early spring where it 

 is found in the pools ere the ice has gone out. All, being 

 unattached (save as they become entangled with rooted 

 aquatics near shore), prefer qidet waters. Immense 

 accumiolations of their tangled filaments often occur on 

 the shores of shallow lakes and ponds, and with the 

 advance of spring and subsidence of the water level, 

 these are left stranded upon the shores. They chiefly 

 compose the "blanket-moss" of the fishermen. They 

 settle upon and smother the shore vegetation, and in 

 their decay they sometimes give off bad odors. Some- 

 times they are heaped in windrows on shelving beaches, 

 and left to decay. 



We most commonly see them floating at the surface 

 in clear, quiet, spring-fed waters in broad filmy masses 

 of yellowish green color, which in the sunlight fairly 

 teem with bubbles of liberated oxygen. These dense 

 masses of filaments furnish a home and shelter for a 

 ntuxiber of small animals, notably Haliplid beetle larvse 

 and punkie larvae among insects; and entanglement 

 by them is a peril to the lives of others, notably certain 

 Mayfly larvae (Blasturus). The rather large filaments 

 afford a solid support for hosts of lesser sessile algae; 

 and their considerable accumulation of organic contents 

 is preyed upon by many parasites. Their role is an 

 important one in the economy of shoal waters, and its 

 importance is due not alone to their power of rapid 

 gro^\'th, but also to their staying qualities. They hold 

 their own in all sorts of temporary waters by develop- 

 ing^ protected reproductive cells known as zygospores, 

 which are able to endtire temporary drouth, or other 

 untoward conditions. Zygospores are formed by the 

 fusion of the contents of two similar cells (the process 



