10 EARLY PROGRESS 



find the secret of their internal structure. Till 

 he can do this, he is like the traveller in a strange 

 city, who looks on the exterior of edifices entirely 

 new to him, but knows nothing of the plan of 

 their internal architecture. To be able to read 

 in the finished structure the plan on which the 

 whole is built is now essential to every naturalist. 

 Each of these plans may be stated in the most 

 general terms. In the Vertebrates there is a 

 vertebral column terminating in a prominent 

 head ; this column has an arch above and an 

 arch below, forming a double internal cavity. 

 The parts are symmetrically arranged on either 

 side of the longitudinal axis of the body. In the 

 Mollusks, also, the parts are arranged according 

 to a bilateral symmetry on either side of the body, 

 but the body has but one cavity, and is a soft, 

 concentrated mass, without a distinct individual- 

 ization of parts. In the Articulates there is but 

 one cavity, and the parts are here again arranged 

 on either side of the longitudinal axis, but in 

 these animals the whole body is divided from end 

 to end into transverse rings or joints movable 

 upon each other. In the Radiates we lose sight 

 of the bilateral symmetry so prevalent in the 

 other three, except as a very subordinate element 

 of structure ; the plan of this lowest type is an 

 organic sphere, in which all parts bear definite 

 relations to a vertical axis. 



