TISSUE-DIFFERENTIATION 117 



ment of a remarkable system of air-tubes, the 

 trachea, which open to the exterior and, permeating 

 the insect body in every direction, permit a direct 

 supply of atmospheric oxygen to get to all the tissues. 

 The lobster presents a next higher step in differ- 

 entiation and specialization. Instead of the elongate 

 dorsal tube there is a boxlike heart, likewise pro- 

 vided with ostia, but opening also into a number of 

 blood tubes (arteries) that carry the blood away from 

 the heart. These arteries divide and subdivide 

 until finally they empty their contents into the open 

 spaces between the muscles and other organs, called 

 sinuses. From these spaces the blood seeps back 

 and, after traversing the gills, reenters the heart 

 through the ostia. Thus a circuit is completed, 

 definite through part of its extent and indefinite 

 through the rest. In vertebrates the blood not only 

 leaves the heart by way of arteries, but is returned 

 to it by another system of tubes called veins, the 

 two systems being connected by much finer vessels 

 called capillaries. Thus the entire circuit is com- 

 pleted within definite channels. Such a system is 

 called a " closed " system in contrast to the " open " 

 system of the lobster. 



Excretory Organs. - - The waste-products men- 

 tioned above are not only of no use to the organism, 

 but, on the other hand, in most cases, are active 

 poisons, whose ill effects are evident if they be ever 

 so slightly concentrated. It is of the highest impor- 

 tance to the animal that these substances be elimi- 



