276 TEXTBOOK OF ZOOLOGY 



nules are tactile and each has two slender filamentous processes, the 

 exopodite and endopodite. In addition to these slender jointed proc- 

 esses each antennule has a saclike statocyst in its coxopodite. This 

 structure is an infolding from the outside and is lined with exo- 

 skeleton and sensory hairs. Inside of each are small particles of 

 solid material, such as grains of sand, which are called statoliths. 

 As the animal changes its position the statoliths move about inside 

 of the statocyst and stimulate the sensory hairs. From these stimu- 

 lations the crayfish is able to determine its orientation in space, 

 i.e., it knows whether it is in normal walking position, on its back, 

 or standing on its head. These organs serve for equilibrium. When 

 the crayfish molts, the statocysts are temporarily lost and new 

 ones form as the new skeleton develops. If there are no solid ob- 

 jects in the water in which a crayfish lives during molting, there 

 will be no statoliths in the statocysts and the ajiimal has an im- 

 paired sense of equilibrium. Experimenters have placed only iron 

 filings in the water at such a time and the animals present have 

 used them for statoliths. By bringing a magnet near the crayfish 

 in this condition the statoliths are moved and the animal goes 

 through numerous peculiar contortions in attempting to respond to 

 these stimulations of orientation. Besides the above functions the 

 antennules provide the chemical senses of smell and taste. 



The eyes, which are of the compound type, are mounted on movable 

 stalks, one on each side of the head region. They are described as 

 compound because each one is composed of a large number of in- 

 dividual sight units, each of which is essentially an eye. Each of 

 these units is called an ommatidium, and the crayfish has about 2,500 

 in its eyes. A single one is rather spike-shaped, tapering from the 

 broader superficial end to the rather pointed internal extremity. A 

 single ommatidium has an outer cornea which is transparent and 

 supported by some corneagen cells on the vitrella. Beneath this is 

 the rather long crystalline cone beneath which is the rhabdom, an- 

 other lenslike structure. Surrounding the latter are sensory cells 

 making up the retinula. The wall of the ommatidium possesses pig- 

 ment cells along the sides of the crystalline cone and in the retinula. 

 The distribution of the pigment varies with the intensity of the light. 

 The stronger the light the more these cells are expanded and the 

 more direct must be the ray of light to reach the retinula, because 



