PROMORPHOLOGY 123 



IV. Promorphology ; the Fundamental Forms. 



The structure of the individual animal depends uj on the definite arrange- 

 ments of its organs, which are definite or only vary slightly in each group. 

 Comparison shows that there are a few fundamental forms which play the 

 same role in morphology as the crystal forms in mineralogy. But there is an 

 important difference. A crystal is made up of similar parts, its form is the result 

 of its physico-chemical composition. This condition cannot exist in animals 

 as each organ is a complex of many chemical compounds. Nor, even where 

 the symmetry is the most pronounced is there that mathematical accuracy found 

 in crystals. 



The form of an animal depends upon its extension in space, and accordingly 

 we may pass through it three axes at right angles to each other, these defining 

 the position of three planes. According to the relations of the body to these we 

 may define five fundamental forms. 



FIG. 88. Sponge, Lophocalyx philippensis, with buds (after F. E. Schulze). 



Asymmetrical (anaxial) animals (fig. 88) are such as have no definite arrange- 

 ment of parts with regard to axes or planes, the body growing irregularly in any 

 direction as in many sponges and protozoa. 



Spherical (homaxial) animals have the parts arranged around a central 

 point, through which innumerable axes and planes may be passed, each plane 

 dividing the whole symmetrically as in some spherical protozoa, chiefly radiolaria 

 (fig. 89). 



In radial (monaxial) symmetry there is a main or longitudinal axis which lies 

 in the direction of growth. It may be longer, shorter or of the same length of 

 the other axes, but it may be distinguished by the fact that it passes through 

 parts, as the mouth, which are not found in other axes. Around this main axis 

 the parts of the body are symmetrically arranged, like the spokes of a wheel, 

 so that any plane passing through the main axis will divide the body into 

 symmetrical parts. Most ccelenterates and echinoderms are more or les: 

 completely radially symmetrical (fig. 90). 



