CRAYFISH 405 



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 animal has an im- 

 paired sense of equilibrium. Experimenters have placed only iron 

 1^ 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 

 t through numerous peculiar contortions in attempting to respond to 

 pthese stimulations of orientation. Besides the above functions the 



antennules provide the chemical senses of smell and taste, 

 p 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 hy 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 

 the possibility of reflection within the ommatidium is reduced. In 

 dim light the pigment is concentrated partly toward the outer and 

 partly toward the basal portion of tl^e ommatidia which allows more 

 refraction of rays by the crystalline cones and a combination of 

 images in several adjacent units. In brighter light only the ray from 

 directly in front of the cornea will reach the retinula and stimulate 

 the nerve cells there. These cells are connected internally with 

 the optic nerve. The type of vision produced in the compound eye 

 is ''mosaic" in that there is registered only a single image by the 

 eye. Each ommatidium which is in focus on the object registers 

 an image of that part. As the object moves, new ommatidia are 



