Chap. 5 



ANIMALS AND THEIR ENVIRONMENTS 



71 



■^i-^llum 



SPHERICAL 



^-tMi0^ 



RADIAL 



ASYMMETRICAL 



BILATERAL 



Fig. 5.5. Types of symmetry. Spherical, a protozoan (radiolarian) floats in water 

 that presses against it equally on all sides; radial, a sea anemone, its shape common 

 in animals that are attached for most of their lives; asymmetrical, in a snail that no 

 plane will divide into halves; bilateral, in a salamander, in animals that move about 

 freely, and are mainly symmetrical on each side of a plane extending the length of 

 the body. 



and the brain and sense organs are always at the end that arrives first. Scarcely 

 any animal is perfectly symmetrical, whatever the type; all tailors know that 

 the human ones are a little one-sided. 



Segmentation. The bodies of all animals from earthworm to man are 

 segmented, i.e., partitioned into sections that are joined together in a series. 

 The segmentation may be conspicuous inside and out, as it is in the earth- 

 worm; it may be mainly on the outside as in the abdomen of an insect; or 

 prominent in certain structures such as vertebrae and ribs. The arrangement 

 has the advantage of making parts of the body more independent of one 

 another; it is an insurance lessening the disaster of injury to the whole body. 

 If one or more segments are hurt, others can carry on. Segmentation gives 

 flexibility to long slender bodies such as those of worms. It allows great variety 

 by the modification of different segments for different functions, as in a lobster, 

 in which some segments bear swimmerets while others bear mouthparts and 

 eyes. 



