216 GRAND STRATEGY OF EVOLUTION 



nity for readjustments and repairs without serious in* 

 terruption to life. 



In general, the body, in animals of this type, is 

 more or less cylindrical, larger toward the head, and 

 tapering toward a growing point at the tail end. It 

 is divided transversely into a series of similar compart- 

 ments, or segments, or metameres, each of which may 

 contain a complete set of all the more important vital 

 organs, unless, it may be, some of them have been 

 locally suppressed by secondary changes. These or- 

 gans, such as eyes, legs, muscle-blocks, and kidneys, or 

 nephridia, appear in pairs, one member on the right 

 side of the metamere, the other in a corresponding po- 

 sition on the left, figs. 5-8. 



In typical cases, the paired organs of various kind: 

 extend in graded, linear series over the whole body, or 

 at least over very considerable portions of it. Tru; 

 members of a given series are often referred to as iden- 

 tical, or homologous organs, serially arranged. 



The alimentary canal, for the reception, consecu- 

 tive treatment, and distribution of food materials, runs 

 lengthwise through the middle of the body; the mouth, 

 or the entrance to the canal, at the head end; the exit, 

 or anus, at the tail end. 



Along one surface of the body is formed the double 

 chain of ganglia and nerve fibres, or the brain and 

 spinal cord, a.a. which constitutes the chief architect- 

 ural landmark in animal morphology, and serves to 

 identify this surface, as the so-called neural surface, 

 whatever position, in reference to gravity, the body may 

 assume in active life. From the pair of ganglia in 

 each metamere are given off, right and left, the pairs of 



