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 
hew 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. 
