FORM AND SYMMETRY. 35 
Finally, as the greater part of embryology consists in study- 
ing the anatomy and histology of an organism at various 
stages of its development, the work of the embryologist 
is also in the main marphological, though he has also 
to inform us, if he can, about the physiology of develop- 
ment. 
Morphology has been defined by Geddes as “the study 
of all the statical aspects of organisms,” in contrast to 
physiology, which is concerned with their vital dynamics. 
In this chapter we shall follow the historical development 
of morphology, and work from the outside inwards. 
I, Form and symmetry.—The form of an animal is due 
to the interaction of two variables—the protoplasmic 
material which composes the organism, and the environ- 
ment which plays upon it. In fact, an animal takes definite 
form just as a mineral does: in both the shape is determined 
’ by the nature of the stuff and by the surrounding influences. 
Activity, or function, also affects form; but function is 
merely action and reaction between the animal and its 
surroundings. 
As regards symmetry, animals may be distinguished 
as—(a) radially symmetrical; (4) bilaterally symmetrical ; 
(c) asymmetrical. 
In a radially symmetrical animal, such as a jelly-fish, the body can 
be halved by a number of vertical planes—it is symmetrical around a 
median vertical axis. That is, it is the same all round, and has no 
right or left side. In a bilaterally symmetrical body, such as a 
worm’s, there is but one plane through which the body can be halved. 
In an asymmetrical animal, such as a snail, accurate halving is im- 
possible. , 
Radial symmetry is illustrated by simple Sponges, most Ccelentera, 
and by many adu/t Echinoderms. As it is the rule in the two lowest 
classes of Metazoa, and as it is characteristic of the very common 
embryonic stage known as the gastrula (an oval or thimble-shaped sac 
consisting of two layers of cells), it is probably more primitive than 
the bilateral symmetry characteristic of most animals above Ccelentera. 
Radial symmetry seems best suited for sedentary life, or for aimless 
floating and drifting. Bilateral symmetry probably arose as it became 
advantageous for animals to move energetically and in definite direc- 
tions, to pursue their prey, avoid their enemies, and seek their mates. 
The formation of a ‘‘brain” is correlated with the habit of moving 
head foremost. Among many-celled animals, some worm type prob- 
ably deserves the credit of beginning the profitable habit of moving 
head foremost. Had some one not taken this step, we should never 
have known our right hand from our left. 
