72 



ZOOLOGY. 



93. Digestion. The alimentary canal is usually with- 

 out convolutions, running straight through the body, 

 from the mouth to the anus. 

 The oesophagus leads to a large 

 stomach, which in some cases is 

 furnished with a calcareous ap- 

 paratus for grinding the food. 

 There is generally a large liver, 

 but no salivary glands have been 

 detected. 



94. Circulation. The heart, 

 when present, is placed above the 

 alimentary canal. In the higher 

 groups, it consists of a single 

 cavity, which, by its contractions, 

 drives the blood through arteries 

 to the various organs, and to the 

 gills, where it is purified by the 

 air contained in the water. There 

 being no capillaries, the blood, 

 after leaving the arteries, is con- 



FIG. 29. CIRCULATION IN THE veyed through the interstices be- 

 tween the organs of the body to 

 the veins which carry it back 

 to the heart. It follows, there- 

 fore, that the heart in this class contains arterial and not 

 venous blood, as in the case of fishes. The heart in some of 

 the lower forms resembles the "dorsal vessel "of the insects. 

 95. Respiration. When the Crustacea possess distinct 

 breathing organs, they always take the form of gills, 

 adapted for obtaining the vital oxygen, not directly from 

 the atmosphere, but from the air contained in water. In 

 the higher groups, these gills are shaped like pointed 

 pyramids, which consist of a central stem, bearing a 

 great number of thin membranous plates placed closely 

 together, but yet admitting a flow of water between 

 them. These organs are abundantly supplied with blood- 

 vessels. In the lobsters they are attached to the bases 



CRAY FISH. 

 aa, anterior artery and 



principal divisions ; 

 ap, posterior aorta. 



heart; 



