OP ANIMALS. 53 



their form, and to insects which they approach in structure, 

 still differing however from both. Their body is elongated, 

 and formed of a generally considerable number of rings, each 

 having one or two pairs of feet. The head is furnished with, 

 antennae and two eyes. Their jaws and mandibles are analo- 

 gous to those of the Crustacea. Their respiration is tracheal. 

 On quitting the egg, the young have six feet and seven or 

 eight rings, the other rings and feet which support them 

 being developed by age. 



45. The crustacea are of all the articulata with articulated 

 feet, the most complex in their organization. The head and the 

 rest of the trunk are sometimes confounded, and at others 

 distinct, they have a tail more or less elongated, divided into 

 segments, and commonly four antennae. The greater part of 

 them have the mouth fitted for grinding and for that purpose 

 are furnished with several jaws, at least six, always lateral. 

 There are always at least fi-ve pairs of feet for motion, whose 

 forms vary according to the kind of movement to be performed. 

 The number of locomotive feet is in an inverse ratio to that of 

 the jaws: in fact the anterior feet approximate to the jaws, in 

 taking their form, and in filling a part of their functions, they 

 can even completely take their place. For respiration they 

 have pyramidal lamellated filamentous and tufted branchiae, 

 generally adhering to the base of some of the feet, or 

 which they even partly replace. Their circulation is double; 

 the blood that has been submitted to the respiratory action, is 

 poured into a great ventral, aortic vessel, which distributes 

 it to all the body, whence it returns by another great vessel or 

 even a true dorsal ventricle, which transmits it to the bran- 

 chiae. They possess a liver, more or less divided, sometimes 

 even into distinct canals, according to the state of the heart. 

 Generation is sexual and oviparous, without a true metamor- 

 phosis. The greater part carry their eggs with them, and 

 they all inhabit the water. They present otherwise a great 

 variety of organization. The jaws, the feet and the branchiae 

 are so nearly allied, that these appendages have all been con- 

 sidered as being of one kind, the first resulting from a trans- 

 formation of the last. The greater part of them have a shell 



