184 THE INDIVIDUAL ORGANISM 



more or less high degrees of organization and integration. Most of the 

 higher Metazoa are comparatively bulky; nearly all have some type of 

 circulatory system; and the nervous system and sense organs are much 

 more complex and highly developed than in the lower types, permitting 

 adaptation to more varied and complicated environments. Many of these 

 higher Metazoa are specialized for a wholly terrestrial existence. Insects, 

 spiders, crabs, worms, starfishes, clams, snails, and the vertebrates 

 (fishes, frogs, reptiles, birds, and mammals) are examples of Metazoa 

 built on the organ-system plan. From among these, the earthworm, the 

 grasshopper, and man may be selected as examples to illustrate relatively 

 simple, moderately complex, and highly complex types of structure at 

 this organizational level. 



DIFFERENT WAYS OF DOING THE SAME THINGS 



We shall now see how some animals representative of different levels 

 of construction carry on some of the fundamental processes necessary for 

 life. For our purposes, it will be sufficient to consider digestion, respira- 

 tion, excretion, and the response to stimuli. The various reproductive 

 methods and breeding habits associated with the different organizational 

 levels will be presented in Chap. XV. 



Digestion 



The purpose of digestion is the same in all animals — i.e., to break down 

 foodstuffs into a form in which they can be taken into the protoplasm 

 of the cell. According to whether this is done by individual cells or by 

 cells acting in concert, digestive methods can be classified under two 

 heads. 



Intracellular digestion. In the Protozoa and the sponges, digestion 

 takes place within the individual cells. Food particles are taken into 

 temporarily formed, liquid-filled spaces surrounded by protoplasm and 

 known as food vacuoles. Digestive enzymes, formed in the surrounding 

 protoplasm, diffuse into the vacuoles and digest the food particles; the 

 products of digestion then diffuse into the protoplasm. Indigestible 

 residues are eliminated from the cell through the cell membrane by a 

 "bursting" of the vacuoles to the outside. 



Extracellular digestion. In all the more complex animals digestion 

 takes place in a body cavity surrounded by digestive cells. Both intra- and 

 extracellular digestion occur in some of the simple metazoa, such as 

 Hydra. All extracellular digestion requires a digestive organ or organ 

 system. The chief types of digestive structures are: 



1. The Coelenteron. A digestive sac, found in very simple metazoa; 

 it has but one opening, the mouth. In the flatworms, the coelenteron is 



