242 



ELEMENTARY BIOLOGY 



Many insect larvae, when they hatch out of the eggs, crawl 

 upward to the tips of the twigs. Many adult insects, when they 

 alight on a tree, assume a position with the head pointing up- 

 ward ; others always rest with the head pointing downward. 

 In some species the position of the insect at rest is determined 



by the source of light 

 rather than by gravity. 



289. Statoliths. In 



the simplest animals the 

 action of gravity is prob- 

 ably similar to that sup- 

 posed to take place in 

 plants, namely, that the 

 nucleus or some other 

 solid particle presses up- 

 on the protoplasm of the 

 cell in a different part, 

 according to the position 

 of the cell. In some ani- 

 mals the organ of equili- 

 bration, or of perceiving 

 gravity, is essentially a 

 hollow space with a float- 

 ing body and sensitive 

 walls (see Fig. 102). 



In the lobster and crayfish similar organs are located at the base of 

 the antennules. The movable body here consists of some grains of sand. 

 These organs were formerly supposed to be related to the percep- 

 tion of sound waves, but it is doubtful whether they function in this 

 manner at all. 



290. Balancing organs. In the vertebrates, organs of equili- 

 bration, or of perception of position, are found in connection 

 with the inner ear. The semicircular canals of the labyrinth 

 (c and k, Fig. 101) are capable of detecting slight movements 

 of the body (or of the head) in any one of the three planes of 

 space (see diagram, Fig. 103). In our own body it is this 



FIG. 103. The three dimensions of space 



A solid body moves in a space which we think of as 

 extending in all directions. Every movement can be 

 thought of as a combination of movements in one 

 or more of the three planes representing the three 

 dimensions of space. The semicircular canals of 

 backboned animals are placed almost at exact right 

 angles to one another 



