FORM AND SYMMETRY. 33 



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 development. 



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. Such statements, however, are platitudes ; 

 we are far from being able to explain the conditions of 

 growth which lead to this shape or that. 



As regards symmetry, animals may be distinguished in 

 an elementary way, as — 



(a) Radially symmetrical ; or 



(b) Bilaterally symmetrical ; or 



(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 our own, 

 there is but one plane through which the body can be halved. In an 

 asymmetrical animal, such as a snail, accurate halving is impossible. 



Radial symmetry is illustrated by simple Sponges, most Ccelentera, 

 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. 

 Among many-celled animals, some worm type probably deserves the 



