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 morphological, 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. 



L 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 ; (ft) 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 Gcelentera, 

 and by many adult 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 nevei 

 have known our right hand from our left. 



