ENVIRONMENTAL CLASSIFICATION 51 



large. The terrestrial animals without a skeleton, such as the land 

 planarians and earthworms, are less different in size from their aquatic 

 relatives. 



The low density of the air is also accountable for the fact that 

 (parasites aside) there are scarcely any sessile terrestrial animals, in 

 contrast with aquatic animals, among which sessile forms are com- 

 mon. To be successful, the sessile habit depends on an abundant food 

 supply, since small animals and organic particles may be passively 

 suspended in the water, and be brought to the animal by the creation 

 of a current, but the air can bear larger particles only in the less fre- 

 quent instances when it is in rapid motion. The lack of a plentiful 

 supply of air-borne food particles also contributes to the necessity 

 for large-yolked eggs among terrestrial animals, since the delicate 

 larvae hatching from small-yolked eggs would not be able to feed 

 themselves. Only when care. of the young takes place and the embryos 

 are supplied with food in the mother's body can the eggs be small and 

 poor in yolk, as in some Peripatus and in the viviparous mammals. 



Fertilization is internal in all air-breathing forms with the single 

 exception of the frogs, which in this respect still behave like aquatic 

 animals. Internal fertilization may take place in aquatic animals, but 

 it is invariable in the fully terrestrial ones. An arrangement like that 

 of wind-fertilized plants, i.e., the transport of male sex cells to the 

 eggs by means of air currents, is conceivable; but this possibility has 

 never been realized. The fertilization of the eggs is usually accom- 

 plished by means of a copulation. This means a great saving in 

 materials on the part of the males, as compared with the free emission 

 of sperm into the water which is so common a process in aquatic 

 animals. This also serves to make the common 1:1 sex ratio, which 

 is definitely adaptive among marine animals with external fertiliza- 

 tion, frequently an anachronism among terrestrial forms. 



A higher development of the organs of sense is a general accom- 

 paniment of the transition to terrestrial life. On account of the greater 

 transparency of the air as compared with that of the water, the eye 

 increases in importance, though no new types of eyes have been 

 evolved. In the terrestrial vertebrates eyes capable of accommodation 

 are adjusted for distance, when at rest, and are actively focused for 

 near objects. In the squids and fishes, when at rest, the eyes remain 

 adjusted for near objects and accommodate actively for the more 

 distant ones, which present faint and indefinite images in water. 



The chemical senses of aquatic animals are developed in the air- 

 breathers into the senses of taste and smell. By this division of labor, 

 the one is stimulated by liquid substances, the other by gaseous ones, 



