1 66 ANIMALS 



well developed in the insect eye. In the vertebrate eye this 

 adjustment is supplemented by a change in the size of the pupil. 

 A circular muscle in the iris causes the pupil to contract while 

 a set of radial muscles cause it to expand. 



HEARING AND EQUILIBRATION 



387. Statocysts. Many of the lower animals have been 

 credited with a sense of hearing, but it is very doubtful whether 

 any aquatic invertebrate has really an organ for perceiving 

 sound. That many aquatic animals may "feel" and respond 

 to vibrations set up in the water is quite probable. But this 



may be due to the stimulation of 

 other organs, such as the tactile 

 sense organs. The organs found 

 in jellyfishes, worms, Crustacea, 

 and many other aquatic inverte- 

 brates, which have been called 

 "ear sacs," are well understood 

 and are more properly called 

 statocysts. 



FIG. 8 3 .-Statocyst of a Mol- 388. In hydra there are no 



hisc. , Nerve; o, otolith; s.c., statocysts, nor are they found in 

 sensory cells. (From Galloway, 



after ciaus.) any other fixed forms. In the 



hydromedusae, however, they are 



very common. They consist, typically, of a deep sack-like 

 depression of the ectoderm, which contains sensory cells and 

 a statolith. The sack may be open or closed, but in either 

 case is filled with a fluid. The sensory cells are provided 

 with bristle-like processes which project into the cavity of 

 the statocyst. The statoliths are heavy concretions of inor- 

 ganic matter which stimulate the sensory cells by contact 

 with the bristles. When the animal turns over in swimming, 

 the statoliths, by their weight, always settle to the lower side 



