246 Adjustment to Conditions of Aquatic Life 



interstices and the interior being filled with 

 gelatinous substances of low specific gravity. 

 /. Dendritic colonies, as in Dinobryon (fig. 32 on 

 p. 106). 



3. In the Metazoa, by the expansion of the external 

 armor and appendages into bristles, spines and fringes. 

 ( Thus in the rotifer Notholca longispina (fig. 149), 

 a habitant of the open water of lakes, there is a 

 great prolongation of the angles of the lorica, 

 before and behind; and in the Copepods (fig. 95, 

 p. 188) there is an extensive development of 

 bristles upon antennas and caudal appendages. 

 Expansions of the body, if mere expansions, 

 serve only to keep the body passively afloat; but 

 many of them have acquired mobility, becom- 

 ing locomotor organs. Cilia and flagella are the 

 simplest of these, and are common to plants and 

 animals. Almost all the appendages of the 

 higher animals, antennae, legs, tails, etc., are 

 here and there adapted for swimming. A body 

 whose specific gravity is but little greater than 

 that of the water may be sustained by a mini- 

 mum use of swimming apparatus. The lesser 

 '^Aiong- flagellate and ciliate forms, both plant and 

 spined animal, maintain their place by continuous lash- 

 rotifer. ^^g ^^ ^^i^ water. If we watch a few waterfleas 

 in a breaker of clear water we shall see that their swim- 

 ming also, is unceasing. Each one swims a few strokes 

 of the long antenna upward, and then settles with 

 bristles all outspread, descending slowly, as resistance 

 yields, to its former level. This it repeats again and 

 again. It may turn to right or to left, rise a little 

 higher or sink a little lower betimes, but it keeps in 

 the main to its proper level. Its swimming powers 

 are to an important degree supplemental to its inade- 



