Phylum Arthropoda 503 



The sense organs of the crayfish are highly developed. The eyes, 

 which are located on moveable stalks below and on either side of the 

 rostrum, are of the compound type, that is, each eye is composed of 

 a large number of individual units, ommatidia. Each ommatidium re- 

 ceives a separate image; thus a mosaic rather than a single image is 

 perceived. Covering the surface of the entire eye is the transparent 

 cornea which is divided by fine lines into a series of four-sided facets, 

 each of which represents the distal portion of a single ommatidium. The 

 ommatidium itself is a complex rod formed of the outer cornea, two 

 inner corneal cells which secrete the cornea, a long cone-shaped crystal- 

 line body fortified by four cells, and two retiniilar cells which surround 

 the crystalline body. In addition, at the base of the ommatidium are 

 seven retinular cells which form a rhabdome, basal pigment cells around 

 the retinida, and finally between the inner bases of the retinular cells a 

 single tapetum cell. All the ommatidia rest on a basement membrane 

 through which the inner end of the retinular cells pass. Each retinular 

 cell connects by means of a nerve fiber to the supraesophageal ganglion. 



There are approximately 2,500 of these individual ommatidia in 

 each eye. In strong light, the pigment cells extend in such a way that 

 each ommatidium is isolated from every other one and a whole series 

 of images is perceived. This type of visual perception is particularly well 

 adapted to the observation of movements. In lower intensities of light, 

 this pigment recedes and the light stimulus is able to pass from one 

 ommatidium to another. This probably results in a continuous image, 

 perhaps somewhat similar to the single image which we perceive, al- 

 though it is undoubtedly quite indistinct. 



At the base of the antennules is another sensory organ, the stato- 

 cyst, which is concerned with balance. This is a chitinized sac lined with 

 a ridge bearing many tiny hairs to which small grains of sand are at- 

 tached by mucus. These hairs are connected by nerves to the brain and 

 detect changes in the position of the body. Proof of this function is 

 easily obtained : a crayfish about to molt is placed in clear filtered water 

 without grains of sand ; when it has completed its molt, it will be noted 

 that its equilibrium is upset. For further proof, tiny iron filings may 

 be placed in the water. These will be placed on the sensory hairs after 

 the molt; then if a magnet is brought near the crayfish, it will orient 

 itself in relation to the magnet instead of to gravity. Further, these 

 experiments demonstrate that the inner portion of the sac with the 

 sensory hairs is lost during the molt, and the tiny grains must be re- 

 placed on the new hairs. 



