322 THE CRAYFISH AND THE ARTHROPODA 



use of this animal in his classic volume entitled " An Introduction 

 to the Study of Zoology, Illustrated by the Crayfish." They 

 abound in the streams and smaller bodies of fresh water in the 

 more temperate regions of North America, although, like other 

 animals, their distribution is determined by past migrations as 

 well as present conditions that may be favorable. In parts of 

 New England they are entirely absent, but throughout most of 

 the Mississippi Valley region, in the Pacific Northwest, and in 

 the southeastern states they are familiar animals wherever a 

 favorable environment presents itself. In many other parts of 

 the world crayfish of various species and genera are likewise 

 abundant. In Europe, in Asia, and in North America west of the 

 Rocky Mountains, are found members of the genus Potamobius 

 (Astacus), while the genus Camharus is typical for the remainder 

 of our continent. Other genera of crayfish are widely distributed 

 in the southern hemisphere, in Australia, Tasmania, New Zea- 

 land, the Fiji Islands, Madagascar, and South America. The 

 first reference to crayfish in scientific literature is found in Aris- 

 totle's writings, where he speaks of the " small astaci that breed 

 in the rivers." Astacus was the Greek name for the lobster, and 

 for a long time the crayfish were known as " river-lobsters." 

 They are, in fact, closely related to the lobsters, although the 

 latter are marine animals and must be placed in a different family. 

 The common name " crayfish " is better scientific usage, although 

 " crawfish " is the popular designation in this country and was used 

 by the naturalist Say, in 1817, years before Huxley gave cur- 

 rency to the English name crayfish in the textbook above men- 

 tioned. 



General External Features. — There are three principal regions 

 of the body, the head, the thorax, and the abdomen (Fig. 155). 

 The body is composed of segments, the somites or metameres, each 

 having a pair of jointed appendages. In the abdomen these seg- 

 ments are in evidence dorsally, but in the head and thorax their 

 dorsal portions are fused together as a continuous structure, the 

 cephalothorax, which shows, however, the line of separation 

 between head and thorax as the cervical groove. The cephalo- 

 thorax is prolonged anteriorly as the pointed rostrum on either 

 side of which are the eyes. Ventral to the eyes are two pairs of 

 appendages, the antennules, which are smaller and biramous; and 

 the antenna, which are more elongated. Posterior to these sensory 



