SOME OTHER TYPES OF INDIVIDUAL ORGANIZATION 189 



3. Lungs as Organs for External Respiration. True lungs are found 

 only in the air-breathing vertebrates. We have seen something of their 

 structure and functioning in man, and, although the lungs of certain 

 of the lower vertebrates are much simpler, they show a comparable 

 structure and have the same function. Some other land animals, such 

 as the spiders and terrestrial snails, also have respiratory organs that 

 function as lungs, although these differ markedly in origin and structure 

 from the lungs of vertebrates. 



Excretion 



Besides excess water and carbon dioxide, there must be removed certain 

 solids in solution, particularly nitrogenous wastes such as urea and uric 

 acid. The methods found in various animals for the excretion of this type 

 of waste are discussed below. 



Excretion in the protozoa. In the Protozoa, excretion doubtless occurs 

 to a greater or less extent through the surface of the body, but usually 

 one or more small contractile vacuoles (Fig. 13.1) concerned with this func- 

 tion are also present. Such a vacuole fills with a watery fluid drained 

 from the surrounding protoplasm and periodically discharges it to the 

 outside, thus washing the protoplasm free from soluble waste materials. 

 In fresh-water Protozoa an equally important function of these cell 

 organs is to "bail out" the water which continually enters the cell as a 

 result of osmosis, and which otherwise would soon cause rupture and 

 death of the organism. 



Excretion in the metazoa. In very simple multicellular animals, 

 the surfaces of the body suffice for excretion ; hence they have no special 

 excretory organs. All except these simplest Metazoa, however, have 

 developed special systems and organs for the removal of wastes. Some 

 of the more important types of these systems are as follows : 



Protonephridial System. This type of excretory system is charac- 

 teristic of the flatworms (Fig. 13.6) and does not require a circulatory 

 system for its operation. The metabolic wastes are carried from the tissues 

 to the outside by means of a system of tubes. This drainage system may 

 be simple or greatly branched, but each tubule originates from a large, 

 cup-shaped cell, the inner wall of which bears a tuft of long cilia. The 

 beating of these cilia suggests the flickering of a flame; hence the cell is 

 called aflame cell. Besides withdrawing fluid from the surrounding tissues, 

 the vibrations of the flame-cell cilia create a current that carries the 

 collected wastes to the outside. 



Nephridia. In the metameric worms (for example, the earthworm, 

 Fig. 13.7) nearly every segment of the body is provided with a pair of 

 coiled tubes, the nephridia, each of which has a funnel-shaped, ciliated 

 opening, the nephrostomy, which projects through the septum into the 



