Chap. 29 ARTHROPODS CRUSTACEANS 583 



bules, and a canal that opens into the urinary bladder which in turn opens ex- 

 ternally. The entire crustacean kidney is in principle comparable to one unit 

 of the vertebrate kidney. Although crayfishes live in fresh water, they keep 

 an adequately salt solution in their bodies by the water resistance of their 

 body cover, by water loss from the kidneys, and by the absorption of salt by 

 the gills. Like all fresh-water invertebrates they contain a higher percentage 

 of salt than the surrounding water which would flood their bodies except for 

 the special means of keeping it out. 



Fig. 29.12. Diagram of the respiratory and circulatory systems of the crayfish 

 or lobster. Efferent blood vessels from gills to heart, and the arteries are unshaded; 

 afferent vessels to the gills, and veins are black. Left side of heart with three open- 

 ings; p, pericardium; h, heart; aa, abdominal aorta; ac, cephalic aorta; as, ventral 

 abdominal artery. (After Claus. Courtesy, Conklin: General Morphology. Prince- 

 ton, Princeton University Press, 1927.) 



Coordination and Response. The central nervous system is similar to that 

 of the earthworm but obviously further developed in the head and thorax 

 corresponding with the more elaborate activities of the crayfish (Fig. 29.11). 



In the embryo, each of the segments contains a pair of ganglia but in the 

 adult crayfish members of the pairs and some of the pairs are fused. Numer- 

 ous nerves penetrate throughout the body, all of them composed of the proc- 

 esses of nerve cells whose bodies are in the ganglia. 



By means of sensory pits and bristles the surface of the body is more or 

 less responsive, the pincers particularly to touch and the antennules to taste. 



The organs of equilibrium by which the crayfish keeps its upright position 

 are located in small chitin-lined sacs, the statocysts, one on the basal segment 

 of each antennule (Fig. 29.11). Within the statocyst is a sensory cushion on 

 which there are numerous sensory hairs innervated by a single nerve cell fiber. 

 Large grains of sand (statoliths) are placed in the cup by the crayfish, an 

 extraordinary habit. These adhere to hairs made sticky by a secretion pro- 

 duced below the sensory cushion. The contact of the sand with the sensory 

 hairs is communicated by way of a nerve fiber to the central nervous system 

 and thence to the muscles. The linings of the statocysts are molted with the 



