250 Adjustment to Conditions of Aquatic Life 



after displacement; and much of the force expended 

 in pushing it aside at the front is regained by the 

 return-push of the water against the sloping rearward 

 portion of the body. 



The advantage of stream-line form is equally great 

 whether a body be moving through still water, or 

 whether it be standing against moving water. A 

 mackerel swimming in the sea is benefited no more than 

 is a darter holding its stationary position on the stream 

 bed. To this we shall have occasion to return when 

 discussing the rapid-water societies. 



Apparatus for propulsion is endlessly varied in the 

 different animal groups. Plants have developed hardly 

 any sort of swimming apparatus beyond cilia and 

 flagella. These also serve the needs of many of the 

 lower animals the protozoa, the flat worms, the roti- 

 fers, trochophores and other larvae, sperm cells generally, 

 etc. But more widely ranging animals of larger size 

 have developed better swimming apparatus, either with 

 or without appendages. Snakes swim by means of 

 horizontal undulating or sculling movements of the 

 body, and so also do many of the common minute 

 Oligochaete worms. Horseleeches swim in much the 

 same manner, save that the undulations of the body are 

 in the vertical plane. Midge larvae (" blood worms") 

 swim with figure-of-8-shaped loopings of the body that 

 are quite characteristic. Mosquito larvae are 

 " wrigglers, " and so also are many fly and beetle larvae, 

 tho each kind wriggles after its own fashion. Dragon- 

 fly nymphs swim by sudden ejection of water from the 

 rectal respiratory chamber. 



All of these swim without the aid of movable appen- 

 dages ; but the larger animals swim by means of special 

 swimming organs, fringed and flattened in form and 

 having an oar-like function. These may be fins, or 



