CONSIDERATIONS. 133 



In the Annelida and Crustacea, where the blood circulates 

 in vessels, we find more or less voluminous conglomerate 

 glands, the largest of which, in the Crustacea, has been con- 

 sidered as the analogue of the liver of Vertebrata ; the second 

 in size, that of the pancreas, or of the kidneys, according as 

 the point of their insertion is near to the gizzard or anus. 

 Those canals whose excretory ducts open into the mouth, or 

 pharynx, may be called salivary glands. 



In Insects, Myriapoda, and the Trachean Arachnida, from 

 the difference of the circulatory system, the blood would not, 

 in such glands, be renewed with sufficient quickness ; the 

 glands, therefore, in these, take the form of long thread-like 

 vessels, which, from their floating in the blood, are easily 

 penetrated by it. This form is not incompatible with a com- 

 plete circulation, for we meet with it in Limulus, and the 

 Pulmonary Arachnida. 



There are from one to five kinds of these, but they are 

 never all present in one species, or at least they are never all 

 apparent. 



The salivary glands are two or four vessels, of varying 

 length, simple or ramose, sometimes having their extremities 

 expanded. 



The glands, to which term biliary vessels is commonly 

 given, are two, four, or six slender, and very long vessels, 

 inserted in different genera, into various parts of the intestinal 

 canalj sometimes above, sometimes below the gizzard. These 

 two extremities sometimes both open into the canal at the 

 same point, sometimes at very distant points. Sometimes 

 their number is very considerable ; they are then either placed 

 in a whorl round a certain part of the canal, or united upon 

 two or more tubercles, placed around one point of the intestine ; 

 sometimes, before their insertion, they all unite into one com- 

 mon duct. 



A third sort of glands, secreting a digestive fluid, may be 

 called gastric glands : these have been mentioned above. 

 When present, they always cover that part of the intestinal 

 canal above the biliary vessels. In the Silphce, the posterior 

 part of the intestines is likewise covered with granulations; 

 these may be called the intestinal glands. 



In many insects, particularly the carnivorous, there exists a 

 fifth kind of gland, the products of which are poured into the 



