366 APPLIED BIOLOGY 



systems : supporting (the external skeleton) ; muscular 

 (muscles of the body and appendages); digestive (mouth, 

 esophagus, stomach, intestine, digestive gland) ; circula- 

 tion (heart, arteries, venous blood-spaces) ; respiration 

 (gills) ; excretion (gills for carbon dioxide, and antennary 

 glands for nitrogenous wastes, and probably some excretions 

 in the indigestible materials discharged occasionally from the 

 intestine) ; nervous (cerebral ganglia above the esophagus, 

 nerve-cords from cerebral ganglia to the ventral nerve-cord, 

 a double nerve-cord in ventral part of the body, and nerves 

 extending to various organs of the body) ; and the reproduc- 

 tive organs (ovaries and oviducts in female, spermaries and 

 sperm-ducts in male). 



If we compare with the frog, we find that the same general 

 functions are represented in the crayfish. 



(1) The supporting of the body of the frog is accomplished 

 by an internal skeleton, while an external case or skeleton 

 serves the same purpose in a crayfish. 



(2) The digestive organs in both animals prepare food for 

 absorption; but the organs for doing this are somewhat 

 different in details of structure. 



(3) Both animals are so large that circulating blood must 

 carry digested food, oxygen, and excretions ; and in each a 

 heart provides the motive force of circulation. The arteries 

 of a crayfish remind us of those of a frog, but instead of 

 tubes or veins for returning blood to the heart, the crayfish 

 has irregular spaces between the various tissues of its body. 

 The heart of a crayfish is very much simpler than that of a 

 frog, being a hollow muscular organ, with valves arranged 

 to allow blood to enter from the pericardial cavity. 



(4) The gills of the crayfish perform the same work as the 

 lungs and skin of the frog. The blood circulating in the 

 capillaries of the gills absorbs oxygen and gives off carbon 

 dioxide, just as does the blood in the lungs and skin of a 

 frog. 



