THE PELAGIAL 229 



ambulaeral feet, which make a free-swimming life possible until their 

 increase in weight outruns their enlargement of surface. In general this 

 increase of surface by projections is not widespread; it appears only 

 among small and very small animals. The great majority of pelagic 

 animals get on without this means of suspension. 



Water resistance, actively secured by means of swimming, is the 

 most widespread means of preventing sinking. Such motion may 

 consist largely of force exerted opposite the direction of gravitational 

 pull as in the pteropods, or may be a small component of lateral 

 motion as in the sharks. The effectiveness of the reaction is measured 

 by the size of the body and its rapidity of motion. The water resistance 

 of a body is proportional to its projection on a plane at right angles 



■ U ''-St. 



Fig. 54. — Juvenile sea urchin (Arbacia pustulosa) : /, ambulacra! feet; p, pedicel- 

 laria; St, spines. After Korschelt and Heider. 



to the line of motion, and to the square of the speed with which the 

 body moves. 



Cilia and lashing hairs, on account of their small surface, are not 

 very effective and are found mainly among protozoans, among very 

 small Metazoa, and among these especially in larval stages. Great 

 numbers of cilia are required to support even small animals. In the 

 almost microscopically small larvae of echinoderms, Balanoglossus, 

 worms, etc., they are arranged in narrow rings or bands. When the 

 larvae increase in size, further support can be gained only by rela- 

 tively great increase in the length of the bands of cilia. The larvae of 

 echinoderms rarely reach any considerable size during their free- 

 swimming stage; but in those which do, the bipinnarians and auricu- 

 larians, reaching a length of about 5 mm., the ciliary bands are 

 folded and bent in a remarkable way (Fig. 55). A more favorable 

 arrangement is the distribution of the cilia over the whole body sur- 

 face, but even so this device is suitable only for small animals. The 

 disk-shaped pelagic turbellarian Haplodiscus has a diameter of only 



