230 



MARINE ANIMALS 



1 mm., and another turbellarian in the North Sea, Alaurina, forms 

 chains 0.9 to 2.5 mm. in length. The ctenophores, whose eight rows of 

 swimming plates are composed of fused cilia of relatively great length, 



Fig. 55. — Auricularia larvae with different lengths of larval life; the largest 

 has the relatively longest ciliary band. Enlarged about 35 times. After 

 Mortensen. 



are successful only by reason of the great reduction of their density by 

 means of gelatinous tissue. 



Among larger pelagic animals, the water resistance is uniformly 

 produced by muscular work. The means used vary. A widespread 

 method consists in the production of a stream of water, by the con- 

 striction of a tube or bell-shaped hollow body, the reaction from the 

 ejected water driving the animal forward and upward. The bell shape 



