278 AN INTRODUCTION TO THE STUDY OF FOSSILS 



The process of molting has been described as follows, a de- 

 scription holding true in general for all Crustacea : " Previous 

 to the throwing ofT of the old skin a new soft one is formed in- 

 side" (the epidermis secretes new layers of chitin), " the lime is 

 absorbed from the old shell in a dorsal line along the carapace, 

 reaching from the rostrum to its posterior margin. Absorption 

 also takes place at the joints of the limbs. The carapace now 

 splits along this dorsal median line of absorption, the blood 

 leaves the limbs, which are thus made flabbier, and by invol- 

 untary muscular movements they are drawn, large claw and all, 

 through the joints of the old shell. The anterior portion of the 

 body is first drawn out through the dorsal vent, and lastly the 

 tail. By means of the return of the blood to the limbs and the 

 rapid absorption of water, the body . . . soon swells to a size far 

 beyond that of the old shell " (2). The lime taken up by the 

 blood by absorption previous to molting is used in hardening 

 the new shell, doubtless aided by the lime carbonate from pieces 

 of shells, sea urchin spines, discarded lining of the stomach, 

 etc., which have been found in the stomachs of crayfish and other 

 Crustacea. 



Beneath the epidermis is a layer of connective tissue, — the 

 dermis to which the muscles are attached. They are thus fas- 

 tened to and supported by the external skeleton. The muscular 

 system consists principally of two longitudinal pairs of muscles, 

 a large and complex ventral pair and a smaller simple dorsal 

 pair, both extending the entire length of the abdomen and into 

 the thorax from whose walls they arise. The dorsal pair or 

 extensor muscles straighten the abdomen, while a portion (the 

 flexor muscles) of the ventral pair bends it downward ; the quick 

 contraction of these latter powerful muscles causes the crayfish 

 to dart rapidly backwards. It moves forward by the action of 

 the abdominal swimming feet. 



Of the five pairs of long thoracic legs the four posterior ones 

 are used in walking, the large anterior one with its huge terminal 

 pincer for defense and offense. Each tube-like section of one of 



