EXTERNAL FORM. 31 



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 in- 

 form 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 in deeper 

 and deeper analysis. 



I. External Form. 



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 crystal 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 al^le to explain the conditions of growth which 

 lead to this shape or that. 



As regards symmetry, animals may be distinguished as: — 



(a) Radially Symmetrical ; 



{b) Bilaterally Symmetrical ; or 



(t') Asymmetrical. 



In a radially symmetrical animal, such as a jelly-fish, the body can 

 l5e halved by a number of vertical planes — it is symmetrical around a 

 median vertical a.xis. 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 Ijody 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 driftng. Bilateral symmetry probably arose as it became 

 advantageous for animals to move energetically and in definite direc- 

 tions, to pursue their prey and avoid their enemies. Among many 

 celled animals, some worm type probably deserves the credit of begin- 

 ning 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. 



