112 MANUAL OF ANIMAL BIOLOGY 



and inasmuch as each one is composed of about 2500 visual units, 

 termed ommatidia, they are known as compound eyes. Each 

 ommatidium is a complicated, elongated, light-receiving structure 

 which is connected at the base with a branch of a nerve from the 

 cerebral ganglion. When this compound eye is viewed from the 

 front, the surface appears as a mosaic made up of a great number 

 of rectangular facets, each of which is the outer end of an omma- 

 tidium. These visual units receive light waves from a restricted 

 area, and so the image which the Crayfish receives is believed to be 

 a composite one, built up from the separate images which have 

 been received by the individual ommatidia. 



D. General Facts of Importance 



The Crayfish belongs to a large and important class of the Arthro- 

 poda, termed the Crustacea, included in which are such common 

 species as the Lobster, Crab, Shrimp, and Spider Crab. In this 

 connection it is interesting to note that the largest known Crus- 

 tacean is the Japanese Spider Crab, shown on page 101. Speci- 

 mens have been found which measured twenty feet across from 

 tip to tip of the outstretched claws of the first pair of legs. They 

 are found at great depths off the coast of Japan. An inter- 

 esting photograph is shown in the National Geographic Magazine, 

 page 72, vol. 54, 1928. On the other hand, many species of small 

 Crustacea, frequently microscopic in size, abound in extraordinary 

 numbers in the fresh and salt waters almost everywhere. Such 

 forms constitute an extremely important source of food for the 

 larger animal types, including the Fishes. 



TEXTBOOK BEFERENCES 

 Woodruff, pp. 90-92 ; 104-109. 



Curtis and Guthrie, pp. 321-341. 

 Guyer, pp. 215-219. 

 Hegner, pp. 250-282. 

 Newman, pp. 231-251. 

 Shull, pp. 87 ; 245-248. 



GENERAL REFERENCES 



Herrick. Natural History of tlie American Lobster (U. S. Bureau of Fisheries, 



1909). 

 Parker and Haswell. Textbook of Zoology (Macmillan). 

 Smith. " Crustacea," in the Cambridge Natural History (Macmillan). 



