THE CRAYFISH. 



135 



On opening a cray- FIG. 73. 



fish or a lobster we 

 find that the interior 

 of the head and of 

 the thorax is more 

 or less regularly di- 

 vided into cavities 

 for the accommoda- 

 tion of the muscles. 

 These divisions are 

 formed by internal 

 processes of the 

 shell that together 

 form a sort of inter- 

 nal skeleton. There 

 are similar processes 

 in the joints of the 

 limbs, where they 

 serve for the attach- 

 ment of muscles. 



The stomach forms 

 a sort of large pocket, 

 separated from the 

 mouth only by a very 

 short oesophagus. 

 This organ is thus 

 brought forward far 

 into the head. In 

 its posterior half its 

 walls are supported 

 by a solid frame- 

 work of joints that 

 support three hard tubercles. These notched tubercles 



CRAYFISH OPENED ON THE BACK. gn, cere- 

 bral ganglion ; era, cesophageal ganglia ; u, 

 green gland; E, stomach; mm, muscles; 

 mg, stomach teeth; F, liver; C, heart; as, 

 posterior aorta; pb, ab, gills; oo, anterior 

 aorta ; ma, muscles. 



