io68 LIFE : OUTLINES OF GENERAL BIOLOGY 



associated with the assumption of more compact shapes in the case 

 of locomotor animals. The spreading bulk of a jellyfish, with a disc 

 over a yard in diameter and tentacles sometimes thirty feet long, 

 could not live except in the open sea. A thirty-foot python or the 

 like might be thought of as contradictory, but although the body is 

 long it is compact, and by the adaptive utilisation of over two 

 hundred pairs of ribs as oars, the nuUipede has become a multipede ! 



EXTERNAL AND INTERNAL SURFACE 



A survey of animal forms shows that two opposite tendencies in 

 organic evolution are towards increase and towards reduction of 

 external surface. In a sedentary arborescent organism, whether 

 Hydroid, Alcyonarian, or Antipatharian colony, a seamat (Flustra), 

 a coralline branching Polyzoon (like Cellepora), there is an enormous 

 surface, and the value of this, familiar in arborescent plants, is that 

 it spreads out the nutritive area. That nutrition, apart from the 

 absorption of soil-water by the roots, is effected by the leaf expan- 

 sion in plants, and by the tentacles and mouths of thousands of 

 polyps in the animal colonies we have mentioned, makes little 

 difference. An Antipatharian or Black Coral colony is sometimes 

 like a small furze bush, and must have a very large tentacled surface, 

 yet without crowding of the individual polyps. It may be contrasted 

 with many of the reef-building or Madrepore corals, where the 

 origin of new individuals, formed by budding or by fission, leads to 

 so much crowding that the younger individuals smother their 

 predecessors. But an arborescent animal colony is like a country 

 with a very long coast-line indented with many fiords, so with 

 available space accordingly. 



While the increase of external surface is most marked in fixed 

 animals, it occurs also in some that float or swim in the water. Thus 

 a large jellyfish, with its many tentacles and its long frilled lips, 

 bears a countless multitude of stinging-cells, whose lassos paralyse 

 and grapple the small animals that serve as food. Interesting in this 

 connection are the Rhizostome jellyfishes, where the usual central 

 mouth disappears and its place is taken by a large number, often 

 several hundreds, of minute openings which lie along the margins 

 of the puckered and growing lips, and communicate more indirectly 

 with the otherwise mouthless food-canal. In many of the free-swim- 

 ming Siphonophore colonies of the open sea, including the Portu- 

 guese Man-of-War and its allies, there is again a very large surface ; 

 but the division of labour is pronounced, and only a certain number 

 of the individuals are concerned with nutrition. 



The adaptiveness of a large external surface in certain conditions 

 of life is emphasised when we consider instances of its marked 



