ARTHROPODA 179 



slightest motion is communicated by the hairs to the 

 cells. 



Moulting of the Shell. — As the shell is hard and does 

 not allow for growth it is moulted or cast off at intervals 

 in order that the animal may grow larger. As soon as 

 the hard coating is off, the animal at once expands ; the 

 tender skin which is thus exposed hardens after a time, 

 and the process is repeated. The new skin repeats every 

 peculiarity of the cast shell so that the surface marking 

 remains the same. Crabs are found in great numbers 

 immediately after they have shed the shell and in this 

 condition are called soft-shelled crabs. Other forms are 

 not found in this condition in sufficient numbers to make 

 them important commercially. 



Digestive System. — On the dorsal surface the ali- 

 mentary canal extends the length of the body. At the 

 anterior end it dips sharply and opens through the 

 mouth on the ventral surface. At the dip is an enlarged 

 stomach with hard walls that grind the food and pass 

 it on into a softer stomach which digests it. If the ani- 

 mal is freshly killed contractions may be seen, pass- 

 ing like a wave the length of the intestine and forcing 

 the contents toward the posterior end. 



Other Organs. — 'Near the anterior end of the lobster 

 are two nephridia-like excretory organs, called the green 

 glands, which give off the waste from the protoplasm. 



Two very prominent brownish glands act as a liver 

 and pour digestive juices into the alimentary canal near 

 its anterior end. 



The heart, situated on the dorsal side, is a curious 

 pentagonal structure with blood vessels passing off from 



