494 GENERAL CONSIDERATIONS 



In the protozoans excretion takes place in part into the contractile 

 vacuole, whence the material is eliminated. In part, however, these 

 wastes escape from the surface of the cell, in which case excretion and 

 ehmination are the same. Many protozoans also accumulate the wastes 

 from metabolism in solid crystalline form in the protoplasm. The 

 material that forms these crystals may be dissolved again and ehminated 

 through the contractile vacuole or the wall of the cell. In the sponges 

 and coelenterates excretion and elimination occur at the same time from 

 the cells. In the echinoderms elimination is through membranes on the 

 surface of the body or lining cavities within the body. There is also the 

 curious form of ehmination carried on by the amebocytes. In echino- 

 derms, too, some excretions are stored as granules and crystals. 



A variety of conditions has been seen in the worms. In the lower 

 forms, such as the planarians, excretion takes place into the watery lymph 

 between the cells and ehmination, through the flame cells. In the higher 

 worms, where a coelom is present, excretion takes place into the coelomic 

 fluid and elimination is by means of nephridia, which are very varied in 

 form and exhibit many degrees of complexity. There are also groups of 

 cells in the coelomic epithelium which take up wastes, escape into the 

 coelom, and then disintegrate, the wastes being passed out through the 

 nephridial tubes. In the mollusks there are both nephridia, known as 

 pericardial glands, and the cells formed from the coelomic epithehum. 

 In the crustaceans are modified nephridia, forming what are known as 

 antennary, or green, glands, opening on the basal segments of the antennae, 

 and in some cases shell glands, opening on the bases of the second maxil- 

 lae; elimination also occurs through the skin, the hning of the intestine, 

 and the liver. In the insects the excretory organs are the malpigUan 

 tubules, which may be considered as being modified nephridia. In the 

 vertebrates, however, three types of kidneys are found— pronephros, 

 mesonephros, and metanephros— which have been previously described 



(Sec. 351). 



516. Motor Functions. — Organisms can in most cases secure food only 

 by moving about in search of it. Other conditions also necessitate loco- 

 motion, and so, even though many animals are sessile and some almost 

 motionless, movement generally is a prominent feature in animal life. 

 Movement has been seen to be of three general types— ameboid; ciUary, 

 or flagellar; and muscular. 



Ameboid movement, which has been noted in the rhizopods, results 

 from a difference in consistency in different parts of the protoplasmic 

 mass. The movements of cilia and flagella, which are similar, are the 

 result of contractions of masses known as basal granules to which the 

 ciha or flagella are attached. Since these granules are in pairs on opposite 

 sides of the base of the cilium or flagellum, movement is produced in only 

 two directions. In metazoans similar granules are connected to an inter- 



