the ostia, which are guarded by valves that allow the blood to 

 enter but prevent it from flowing back into the sinus. 



The nervous system is similar to that of the earthworm and 

 the insects. It consists of a dorsal ganglionic mass, the brain, 

 which is connected by cords passing around the esophagus 

 with the ventral nerve cord which lies along the floor of the 

 thorax and abdomen. Along the ventral nerve cord are a 

 number of ganglia which give off nerve fibers that reach the 

 various parts of the body. 



The abdomen is quite filled with the powerful muscles that 

 flex that part of the body forward when the crayfish is swim- 

 ming, thus producing backward locomotion. These muscles, 

 as well as the strong muscles of the appendages, are all at- 

 tached to the firm body-wall, or exoskeleton, not to an 

 internal skeleton as in the vertebrates. 



The sexes are separated. The spermatozoa are produced in 

 the male in the tri-lobed testis which lies under the heart and 

 over the anterior end of the intestine. They pass through the 

 long, coiled, paired vasa defer entia to the genital apertures in the 

 base of the last pair of thoracic legs. The eggs arise in the 

 bilobed ovary in the female and pass through the paired ovi- 

 ducts and out through the genital apertures on the next to the 

 last pair of thoracic legs. The eggs are glued to the swimmerets 

 of the female, and remain in this position until they hatch. 



During the very early part of their development the young 

 or larval crayfish cling to the old egg shells or to the spinnerets 

 of their mother thus receiving much needed protection, for 

 their bodies are very soft. After about two days the young 

 make their first moult, casting off the old skin after a new one 

 has been formed underneath it. They usually moult about 

 seven times during the first summer and grow very rapidly 

 after each moult. In the process of moulting the lining of the 

 esophagus, stomach and the alimentary canal is also cast off. 

 Often one or more of the appendages may be lost during 

 moulting. Such lost parts are usually regenerated and it is 

 not uncommon to find individuals with one of the claws or 

 legs shorter than the others, the short appendage being a new 

 one that is replacing one that has been lost. 



