THE ARTHROPODS 365 



gus these two nerve-cords unite to form the double nerve-cord, 

 which continues through a sort of tunnel in the ventral part of the 

 thoracic skeleton to the part of the nerve-cord already seen in the 

 abdomen. In this thoracic part of the nerve-cord there is a ganglion 

 opposite each pair of appendages. Each ganglion and pair of ap- 

 pendages represent a segment. 



Respiratory Organs. As in the frog, respiration of crayfish 

 includes taking in oxygen and excreting carbon dioxide. There is 

 simple absorption of oxygen from the water touching the gills (bran- 

 chiae) by the blood circulating inside the gills, and absorption of 

 carbon dioxide from the blood by the water. The delicate membranes 

 covering the gills are well adapted to such passage of the two gases 

 between blood and water separated by the thin membranes through 

 which a kind of osmosis of the gases occurs. 



Obviously, water from outside must continually be coming to the 

 gills, otherwise the available oxygen dissolved in the water would 

 soon be extracted and the carbon dioxide would become excessive in 

 amount. To prevent this, the water in the gill-chamber is continually 

 being changed, entering around the legs and at posterior edge of the 

 carapace and being "scooped" out by a peculiar appendage at the 

 anterior end of the chamber, near the mouth-parts. 



Excretory Organs. The gills, as already mentioned, are the 

 excretory organs for carbon dioxide. The excretion of nitrogenous 

 waste is performed by two organs which lie in the anterior part of 

 the body and just above the bases of the large antennae. On each 

 antenna note near its attachment to the body a small white tubercle 

 with a central opening. Into this opening inject some colored 

 fluid (using a sharp-pointed pipette), and note the organ into which 

 the fluid goes. This is the antennary gland (a simple kidney). 

 Notice that a similar organ lies above the antenna on the other side 

 of the body, i.e., there is a pair of these glands bilaterally arranged. 

 Owing to their color in fresh specimens, they are sometines called 

 "green glands." Blood circulating in the tissues of these glands 

 gives out some of the nitrogenous wastes brought from the other 

 organs of the body, just as blood in the kidneys of the frog gives off, 

 through the cells, such wastes into the tubules which lead into the 

 larger ducts of the kidneys. 



311. Physiology of the Crayfish. The crayfish has organs 

 much more highly differentiated than those of the earth- 

 worm, but less than those of the frog. In studying the struc- 

 ture of the animal, we have found organs of the following 



