26 THE ELEMENTS OF STRUCTUHE. 



countless years ago, and dissecting a modern frog. The 

 anatomical palaeontologist is also a student of morphology. 

 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 of course he ought 

 also to inform us, if he can, about the physiology of develop- 

 ment. 



Morphology, then, may be defined 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 mor- 

 phology, and work from the outside inwards in deeper and 

 deeper analysis. 



I. External Form. 



I have not to speak of the beautiful shapes of animals, 

 nor of the manner in which form is adapted, for instance in 

 fishes and birds, to the conditions of life ; I call your atten- 

 tion to more commonplace but essential facts. Sponges and 

 Stinging-animals have an entirely different symmetry from all 

 other Metazoa. For while most many-celled animals pass in 

 early life through an embryonic stage called the gastrula— 

 an oval or thimble-shaped sac of two layers of cells — the 

 Sponges and Coelenterates alone preserve the symmetry 

 of this stage. In a simple tubular sponge, in Jfydra, or 

 in a sea-anemone, the oral-aboral or long axis extend- 

 ing from the mouth to the other pole of the body, cor- 

 responds to the long axis of the gastrula. Round this axis, 

 moreover, the simple sponge and almost every coelenterate 

 is radially symmetrical. It is the same all round. In other 

 animals, however, the long axis of the body, the oral-aboral 

 axis, say of an earthworm or a fish, does not correspond to 

 the longitudinal (but rather to the transverse) axis of the 

 gastrula. Moreover, these animals are bilaterally not radi- 

 ally symmetrical. They have a head and a tail, and two sides. 

 Some worm-like animals seem to have begun the profitable 

 habit of moving, head foremost ; had they not done so, we 

 should never have known our right hand from our left. The 



