CRUSTACEA 51 



in sheltered nooks waiting for some unwary fish or insect 

 to swim by. They catch such animals in their big pin- 

 chers, or chelipeds, and tear them to pieces. If living 

 animal food fails, crayfishes eat plants, or any sort of dead 

 organic matter and are hence good scavengers. Fine 

 particles of food are retained by the little bristles along the 

 margins of the chelipeds and the mouth parts, so that noth- 

 ing of value is lost. 



A crayfish is well equipped with sense organs to find its 

 food. There are two pairs of antennae at the anterior end 

 of the body and the shorter pair, the antennules, is not only 

 provided with sensory bristles for receiving touch stimuli, 

 but possesses organs of smell as well. A crayfish is able 

 to taste with the nerve endings on the mouth parts. The 

 compound eyes are set on stalks so that they can be moved 

 about in different directions. Each is made up of hundreds 

 of minute but complete eyes. Together these little eyes 

 permit a crayfish to see a complete but somewhat broken 

 picture of its surroundings a mosaic image. If a crayfish 

 is blind in one of its simple eyes, there is a space in its field 

 of vision where nothing is visible. 



Once the food is secured, it must be converted into living 

 substance or modified so as to serve as fuel to run the living 

 machine. It is pulled apart by the chelipeds, torn into 

 shreds by the mouth parts, then passes down into the stom- 

 ach, where it is further chewed by three strong projecting 

 teeth and strained between little bristles. It is mixed with 

 the digestive secretion from the " liver" and finally absorbed 

 through the walls of the intestine. What cannot be digested 

 and absorbed passes out of the anus as faeces. The digested 

 food which passes through the wall of the alimentary canal 

 enters great blood spaces, the sinuses, and ultimately reaches 

 the small muscular heart, in the middle of the back, which 

 pumps blood to all parts of the body. The heart works only 

 when it contracts. It is passive during expansion, being 

 stretched out by little elastic strands which lead from it to 

 the walls of the chamber in which it lies. The blood of the 



