78 



THE ORGANIZED ANIMAL 



endoderm 



Fig. 4-10. Different types of cavities in various animals. 



are important in describing the location of 

 organs and other parts of the animal. 



A small number of animals, some Pro- 

 tozoa and early embryos of higher forms, 

 exhibit universal symmetry. In such animals 

 it is possible to draw any number of planes 

 passing through a central point which will 

 divide the animal into equal halves. Obvi- 

 ously such forms are spheres, such as Vol- 

 vox (Fig. 4-9). 



In other animals it is possible to draw a 

 number of planes through a central line or 

 axis, dividing the animal body into equal 

 halves. These animals are said to possess 

 radial symmetry. Hydra has this type of 

 symmetry. Radial symmetry is usually con- 

 fined to lower animals, for such forms are 

 more or less helpless in doing much about 

 their surrounding world. These animals are 

 usually sessile, though some move very 

 feebly, and they must wait for their food 

 to come within their grasp. In other words, 

 unable to pursue and apprehend tiieir food, 

 they are passive animals that survive only 

 because of the richness of their food laden 

 environment. As animals have become more 

 complex, this type of symmetry becomes 

 unsatisfactory and gives way to bilateral 

 symmetry which provides the animal with 



a body plan that is conducive to further 

 development both in size and complexity. 



Some animals, including many Protozoa, 

 are without symmetry, that is, they are 

 asymmetrical; there is no way of dividing 

 them into halves by means of a plane. Ani- 

 mals may be symmetrical externally but 

 asymmetrical in regard to their internal 

 organs. Man is a good example of this. Ex- 

 ternally he is symmetrical, but his liver, 

 stomach, spleen, and heart are asymmetri- 

 cally arranged. Some fish such as the floun- 

 der are asymmetrical as adults but sym- 

 metrical during their immature stages. A 

 very young flounder looks just like any 

 other young fish, but as it matures one eye 

 misirates through the head so that both 

 of them come to lie on one side and the 

 fish lies flat on the bottom on one side. This 

 is an apparent advantage in survival for 

 this peculiar animal. 



Body cavities. The formation of cavities 

 is characteristic of the earliest metazoan 

 forms. Such lowly forms as Volvox have a 

 central cavity in which the young develop. 

 Among the animals such as Hydra a more 

 useful cavity has evolved, one which func- 

 tions in the storing of food during digestion. 

 Such a cavity is referred to as a coelenteron 



