538 EVOLUTION OF ANIMALS Part V 



of the Pacific Coast. The currents of cilia that deliver the fragments of food to 

 the clam's mouth at the same time serve it to the worms. Certain ribbon worms 

 (Carcinonemertes) live on crabs, as larvae on the gills and as adults on the 

 eggs. In the water as on land every cranny is a home niche for a plant or 

 animal. 



Phylum Rotifera — Trochelminthes 



Their Place in Nature. Rotifers are minute animals that abound in fresh 

 waters throughout the world. Only a few species live in salt water. Their forms 

 and habits are in general similar, but in details they are varied and fit with 

 durable nicety into the niches that their worldwide distribution supplies. They 

 consume microscopic plants and animals, living or dead, and clean the water 

 of its least debris. In turn they are eaten by their next sized neighbors, which 

 are in turn eaten by larger animals and so on up to the fishes. 



The body walls of rotifers are transparent, and their internal organs as easy 

 to see as the works of a glass clock. Both inside and out their constant activity 

 is visible. One of the early observers (Eichhorn 1761) wrote of Floscularia 

 (Fig. 27.5), "Now I come to a very wonderful animal, which has very often 

 rejoiced me in my observations: I call it the Catcher: extraordinarily artistic 

 in its structure, wonderful in its actions, rapid in capturing its prey." Anton 

 Leeuwenhoek, who first described rotifers in 1703, gave them their name 

 meaning wheel-bearers and thought that he had discovered the principle of 

 the rotating wheel in nature. 



Rotifers are an old group with a very long evolution. They resemble flat- 

 worms in having ciliated excretory organs called the flame cells. As in round- 



FiG. 27.5. Representative rotifers selected from the great variety of form and 

 habit in this typically fresh-water group. Floscularia (old name Melicerta), 

 Rotifers of this group create tubes of a gelatinous secretion covered with 

 meticulously fashioned pellets and attached to submerged plants. They are ele- 

 gant but precise creations — minute, yet easily visible to the naked eye. Asplanchna, 

 a predator on all kinds of minute animals including other rotifers. Brachionus 

 is an omnivorous eater, dependent upon its crown of cilia to whirl particles of 

 food into its grinding mastax. Polyarthra, a lake rotifer often in water that is poor 

 in oxygen. It moves by jerks owing to the sudden beating movements of its 

 long appendages. (Adapted from various sources. Courtesy, Needham and Need- 

 ham: Guide to the Study of Fresh Water Biology. Ithaca, N.Y., Cornell Uni- 

 versity Press, 1941.) 



