180 PHYSIOLOGY 



the corners. On its surface are large openings through 

 which the blood re-enters the heart from the spaces into 

 which it is poured. The blood is uncolored. It very 

 quickly coagulates if a vessel is cut. 



Underneath lie the large muscles used for food. They 

 almost fill the abdomen and their segmental arrangement 

 is evident at a glance. Their contraction gives the abdo- 

 men the powerful stroke by means of which a passage is 

 forced through the water. 



Respiration. — As the lobster is covered with a hard 

 shell, a specialized moist membrane has been set aside 

 for breathing. This membrane is confined to the sur- 

 face of the gills, which lie under the free edge of the 

 shell. Water is kept running over them by an appen- 

 dage which moves in such a way that it causes a 

 current. Oxygen is thus able to pass from the water 

 through the thin gill membrane into the blood and 

 carbon dioxide is able to pass from the blood into the 

 water. 



Reproduction. — The reproductive organs are very 

 prominent, especially in the breeding season. The sexes 

 are separate. The eggs pass out of the body through 

 ducts that open usually on the base of the third (fe- 

 male) or fifth (male) walking foot and are held by the 

 swimmerets, or appendages of the abdomen. While there 

 they come in contact with sperm and begin their develop- 

 ment. They remain there until the eggs are hatched 

 and the young can shift for themselves. The little 

 larvae are unlike the parent but soon undergo the change 

 which gives them the adult form. 



Segmentation. — The nerve cord lies on the ventral 



