الفهرس 
IntroductionOnly 14 pages are availabe for public view
 |  | from | 11 |  |  |
| | | from | 11 | | |
Abstract
Procambarus clarkii :Over three hundred kinds of crayfish occure in North America. Each crayfish has specific habitat requirements and is thus limited in its range. However, since there are so many species, crayfish can be found in almost every imaginable aquatic or semi-aquatic environment throughout most of southern Canada and the United States Crayfish-sometimes called crawdads, mudbugs, or crabs-are freshwater organisms related to marine shrimp, crabs and of course, lobsters, which they closely resemble. All are members of a group called Crustacean, characterized by a hard exoskeleton, numerous legs and the presence of gills. These, in turn are members of a larger group, the Arthropod, which includes insects and spiders and is the largest group of animals on earth.
The crayfish Procambarus clarkii is a newly invasive species accidentally introduced in 1980 to Egypt, which has rapidly become established along the River Nile. Crayfish have been widely used as a bioindicator for detecting the effect of environmental changed (Anderson et al., 1997; Zaranko et al., 1997).
The characteristic body of the crayfish can be divided into three parts: the head and the thorax without apparent segmentation and are combined into a cephalothorax, while the abdomen is clearly segmented. The entire length of the body is composed of 20 somites (six in the head, eight in the thorax, and six well defined in the abdomen). The somites of the cephalothorax are coverd by a hard shield or carapace that encloses the back and sides. A cervical groove demarks the head from the thorax .the abdominal somites contain a transverse back or dorsal plate called tergum.