226 



THE CRAYFISH 



circles the gullet and forms a brain in the head region, the latter formed 

 from several ganglia which have grown together. From each of the 

 ganglia, nerves pass off to the sense organs and into the muscles of the 

 body. These nerve fibers are of two sorts, those bearing messages from 

 the outside of the body to the central nervous system (these messages 

 result in sensations), and those which take outgoing messages from the 

 central nervous system (motor impvilses), which result in muscular move- 

 ments. 



Development. — The sexes in the crayfish are distinct. The eggs are 

 fertilized by the sperm cells as they pass to the outside of the body of the 

 female. The developing eggs, which are provided with a considerable 

 supply of food material called yolk, are glued fast to the swtmmerets of 

 the mother, and there develop in safety. The young, when they first 

 hatch, remain clinging to the swimmerets for several weeks. 



Excretion of Wastes. — ■ On the basal joint of the antennse are found two 

 projections, in the center of which are found tiny holes. These are the open- 

 ings of the green glands, organs which have the function of the elimination 

 of nitrogenous waste from the blood, the fuiiction of the human kidneys. 



North American lobster. This 

 specimen, preserved at the 

 U.S. Fish Commission at 

 Woods Hole, was of unusual 

 size and weighed over twenty 

 pounds. Notice the chelipeds. 



/ The North American Lobster. — In 



structure it is almost the counterpart 

 of its snaaller cousin, the crayfish. Its 

 geographical range is a strip of ocean 

 bottom along our coast, estimated to 

 vary from thirty to fifty miles in width. 

 This strip extends from Labrador on 

 the north to Delaware on the south. 

 The lobster is highly sensitive to 

 changes in temperature. It migrates 

 from deep to shallow water, or vice 

 versa, according to the temperature of 

 the water, which in winter is relatively 

 warmer in deep water and cooler in 

 shallows. Sudden changes in the water 

 of a given locality may cause them to 

 disappear from that place. The more 

 abundant food supply near the shore 

 also aids in determining the habitat of 

 the lobster. Lobsters do not appear 

 to migrate north and south along the 

 coast. While little is known about 



