198 GENERAL ZOOLOGY 



suggested that the visual purple increases the irritability of the 

 rods in dim lights. The domestic fowl, the pigeon, and some 

 reptiles lack visual purple. It is present in the eye of the frog. 



Sound Receptors of Invertebrates. — Among invertebrates the 

 only group in which organs of hearing are known with some 

 degree of certainty are the insects. Since some insects are pro- 

 vided with structures for producing sounds, it is reasonable to 

 assume that they also have means of perceiving sounds. Thus 

 grasshoppers produce a sound by rubbing the femur of the last 

 pair of legs against the anterior wings. (The femur is the large 

 segment of the leg near the body.) In grasshoppers, the organs 

 of hearing are thought to be the tympanal organs, located one on 

 either side of the body where the abdomen joins the thorax. 

 The tympanal organ consists of a thin chitinous membrane, 

 stretched over a shallow cavity and provided with nerve endings. 

 Sound waves impinging upon the membrane cause it to vibrate 

 and stimulate the nerve ending. Similar organs are found on 

 the anterior legs of crickets, katydids, and ants. 



Gravitational Receptors of Invertebrates.— Gravitational 

 receptors provide an animal with a means of maintaining its 

 balance or equilibrium. Organs of this sort, known as stato- 

 cysts or lithocysts, are found in Crustacea, such as the crayfish, 

 lobster, shrimp, etc. The statocyst is a sac, located in the basal 

 segment of each antennule, open to the outside, and lined with 

 chitin. Projecting from the inner surface of the sac are sensory 

 hairs among which are found a few grains of sand, called stato- 

 liths. Changes in position of the body displace the statoliths 

 among the hairs, the differential stimulation of which gives the 

 animal a sense of its position in space. This interpretation of the 

 function of the statocyst is borne out by the fact that when 

 the animal molts — a process in which the entire chitinous covering 

 of the body, including the lining of the statocyst and the con- 

 tained statoliths, is lost — the animal for a time lacks full power 

 for maintaining itself right side up. The most convincing evi- 

 dence regarding the function of the organ comes from an experi- 

 ment in which shrimps newly molted and therefore without 

 statoliths were placed in water containing iron filings. In the 

 absence of sand grains to replace the discarded statoliths, the 

 experimental animals placed iron filings in the statocysts. When 

 an electromagnet of sufficient strength was brought near, the 



