COMPARATIVE MORPHOLOGY. 



CHAPTER III. 



COMPARATIVE MORPHOLOGY. 



IN 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 Pro-mcrpkology, and in the second we investigate 

 the internal construction of organisms. On a 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 : — 



2; Aro^'symC^^^^ } ^^^^^^ ^^'""^^'^ ^"^^'^^^^^^ symmetrical). 



3. Piano-symmetric (bilaterally symmetrical). 



1. Centro-symmetric animals have all their parts 

 arranged about a point in the centre of the body, hence 

 they are usually spherical or stellate. The only parts to be 

 distinguished are central and peripheral. This form of 

 symmetry is only found in the lowest aquatic animals {e.g.^ 

 Radiolaria, Foraminifei-a^ &c., and many eggs). 



2. Axo-SYM METRIC auimals have their organs arranged 

 about an axis down the centre of the body and hence they 



