2^8 ON THE CLASSES OF THE 



membranaceous, where the rows of teeth on the tongue are 

 disposed in such a manner on each side as to lose altoge- 

 ther that horizontal position of the organs of mastication, 

 which is so constant throughout the Vertebrata, and 

 where they take a structure more analogous to what we 

 shall soon see to be a character of the Annulosa. 



The common Leech (Hirudo medichialis Linn, or 

 Hamopis medicinalis of Savigny) is a red-blooded aquatic 

 animal, which swims like lampreys with an undulatory 

 motion. Like these animals it has a circulation com- 

 posed of veins and arteries. It breathes like them by two 

 -rows of holes which communicate with branchial pouches. 

 The mouth is still surrounded by a lip proper "for suction, 

 and contains three maxillae, one answering to the solitary 

 upper tooth of the Gastrobranchus, and the others to the 

 lateral teeth of its tongue. These maxillae are minute, 

 compressed, and serrated with very small teeth. The 

 affinity of the leech to the cyclostomous fishes seems to 

 have been first perceived by Linnaeus, as appears by the 

 place which he has given to his genus Myxine. When 

 moreover we find their habits to be so similar, it may be 

 said that nothing more is wanted to complete the resem- 

 blance, than that the wrinkled membranaceous skeleton of 

 Ammocatus should in the leech be supposed to form the 

 envelope for the whole animal. 



A great alteration has however now taken place in the 

 internal structure, notwithstanding the above very evident 

 affinities ; and in order to develope it we may proceed to 

 consider generally the 



Annelides, 

 or Fefs xi sang rouge of Cuvier. To this gentlemati we 



