1070 LIFE : OUTLINES OF GENERAL BIOLOGY 



a noteworthy change had to come about — the proportionate reduc- 

 tion of the external surface. The possibiUty of cutaneous, or, in any 

 case, external respiration, so characteristic of aquatic animals, was 

 greatly reduced; the skin was thickened, and in rfiany cases protected 

 from drought and glare and attack, by cuticle and shell, by scale and 

 scute. It was profitable to have the surface reduced; and we have 

 already referred to the need for compact bodies in typical terrestrial 

 animals. 



Yet this led to the great development of well-packed bodies and 

 of internal surfaces. Thus, taking the respiratory function, we see 

 the need for new structures like insect trachese, arachnid lung-books, 

 or plaited pulmonary sacs. Oxygen was more abundant than it had 

 been in the sea, but it was less available, for the external surface 

 was hardened, protected, and tending to be dry; hence the evolu- 

 tion of increased internal respiratory surfaces. Variations in this 

 direction would tend to succeed; all the more since the reduction 

 of the external surface would tend to impose, as a condition of 

 increase, the formation of internal branches, plaits, and pouches — 

 all making for efficiency. And what is clear in regard to respiratory 

 organs holds also for other systems, such as food-canal, blood-vessels, 

 and kidneys. 



We are not forgetting that some aquatic animals also show great 

 increase of internal surface. Thus there is the extensive superficial 

 area of the canal-system in the more complex sponges, of the gastro- 

 vascular system in jellyfishes, of the alimentary system in a Sea- 

 Mouse (Aphrodite), of the excretory system in a Turbellarian, and 

 so on. Our point is simply that the transition from water dry land 

 gave a fresh impetus to the evolution of internal surfaces. It is 

 highly improbable that the convolutions of the clever mammal's 

 brain, implying a great increase of neural surface without corres- 

 ponding increase in the size of the enclosing brain-box or skull, 

 could ever have come about if animals had not made the great step 

 of leaving the water for the dry land. 



ADAPTATIONS IN SYMMETRY 



Radial symmetry is familiarly illustrated (Fig. 185) by jellyfishes, sea- 

 anemones, corals, and the like, by the simple vase-like sponges, 

 and by the sea-urchins and starfishes, which Cuvier gathered 

 together as his Radiata. We have discussed it under Morphology, 

 so that we need only say here that it implies the possibility 

 of dividing the animal's body into several mirroring halves 

 by cuts in as many different planes. In other words, there is prac- 

 tically no right side or left side in a polyp any more than there is 

 in a glass tumbler. It must be noted, however, that when details 



