22 COMPARATIVE MORPHOLOGY. 
CHAPTER III. 
COMPARATIVE MORPHOLOGY. 
N studying the structure of an organism, we can re- 
cognise two departments of morphology. In the 
first we have to deal with the form assumed by the 
organism or “‘ body-form,” the study of which is sometimes 
termed Fro-morphology, and in the second we investigate 
the internal construction of organisms. Ona first inspection 
of typical examples of animals their body-form does not 
appear to be referable to any definite plan. They do not 
assume geometrical shapes, like a cube, or a cylinder, and so 
on. Yet we can, especially by a study of the lower types, 
find geometrical principles underlying their construction. 
A like environment produces a similarity of structure in 
response to it. For example, if an animal exposes two 
sides to a similar environment, the structure of these two 
sides will tend to be similar. 
The manner in which the similar parts of an organism 
are arranged is termed its symmetry. 
Animal Symmetry.—Animals may be divided into 
three groups according to the symmetry of their body :— 
4 oe } (often termed radially symmetrical). 
3. Plano-symmietric (bilaterally symmetrical). 
1. CENTRO-SYMMETRIC animals have all their parts 
arranged about a jozt in the centre of the body, hence 
they are usually spherical or stellate. The only parts to be 
distinguished are central and feripheral. This form of 
symmetry is only found in the lowest aquatic animals (eg., 
Leadiolaria, Foraminifera, &c., and many eggs). 
2, AXO-SYMMETRIC animals have their organs arranged 
about an axis down the centre of the body and hence they 
