GENERAL CONSIDERATIONS 



23 



Fig. 2. — Red Coral {Corailiutji rubruni] mag- 

 nified. A, Polypes; b and c, Embryos. 



expression of the fact that the external influences which act upon 

 the body are different in different directions, and we must seek in 

 this fact an explanation of the origin of such symmetry. Were 

 external influences the same in all directions we might expect the 

 body to be spherical, but as this 

 obviously can rarely or never be the 

 case, we are prepared for the fact that 

 absolutely spherical animals scarcely 

 exist. In certain fixed forms, how- 

 ever, as coral polypes (fig. 2) and the 

 like, we do get radial or star-like 

 symmetry, where there is a distinc- 

 tion between upper and lower, but 

 none between right and left, or pos- 

 terior and anterior. In such a case 

 the part of the animal facing up- 

 wards is exposed to the light, while 

 the part facing downwards is shaded. 

 All parts of the margin, however, 

 are, on the whole, liable to be acted 



on in much the same kind of way — by light and other external 

 influences. 



Limbs. — We further notice, as regards a fish, that, in addition 

 to the unpaired expansions or fins, which project from the middle 

 line of the body, there are four paired fins, which may with pro- 

 priety be termed limbs, i.e. a pair of anterior or pectoral fins and 

 a pair of posterior or pelvic fins. These are all unjointed, 

 paddle-shaped expansions, with quite continuous margins. Let 

 us now turn to the Frog, which begins life as a fish-like tadpole. 

 Fixing our attention upon an individual which is beginning to turn 

 into a frog, we shall distinguish all the features which belong to the 

 bilateral state ; but the limbs, instead of being fin-like, are divided 

 transversely into certain regions, and are also provided with digits. 

 Both of these features have relation to the semi-terrestrial life 

 which the adult is destined to lead. Taking next an adult frog, 

 we find the limbs well-developed, while the long swimming tail 

 which was so conspicuous a feature in the tadpole has entirely 

 disappeared. Nevertheless the trunk clearly possesses anterior 

 and posterior ends, while the distinction between dark dorsal and 

 light ventral surface is equally clear. Suppose the frog now to 



