1072 



LIFE : OUTLINES OF GENERAL BIOLOGY 



and they swim in the ordinary position near the surface. As they 

 grow larger, one side becomes heavier than the other, and the young 

 fishes swim obliquely at some distance below the surface. Gradually 

 they sink to the floor of the sea, resting and swimming, usually on 

 their left side (plaice, sole, lemon sole, dab, etc^), sometimes on their 

 right side (brill, megrim, turbot), whichever is the heavier. The 

 down-turned eye travels to the upturned side and lies beside its 



i 



Fig. 188. 



One of the Sea-Pens or Pennatulas, a typical deep-water animal. From a 

 specimen. The stalk is embedded in the substratum. The pinnae bear 

 numerous nutritive and reproductive polyps or autozooids, and besides 

 these there are numerous siphonozooids, which are neither nutritive nor 

 reproductive, but serve to keep up currents of water in the colony. 



neighbour; pigment disappears from the unillumined down-turned 

 side, which shows only silveriness, — due to spangles of the waste- 

 product guanin. The asymmetry is adaptive to life on the floor of 

 the sea. 



Stream-lines. — A structural adaptation in which the functioning 

 of the individual has a formative share is a shape suited to a particu- 

 lar kind of locomotion. Thus the torpedo-like form of body in most 

 fishes is adapted to rapid swimming, and similar stream-lines are 

 seen in whales and dolphins, in Sirenians and seals. The serpent-like 



