126 M. straus-durckheim's 



contrary, how great are her resources, and what profound 

 wisdom does she exhibit in varying and combining them 

 without ever creating any thing which is not in the most per- 

 fect harmony ! 



The differences which we have been remarking are prin- 

 cipally due to the gradation in structure of the digestive 

 organs, and the anomalies they present, which are often only 

 apparent, depend on causes unconnected with the digestive 

 system, as, for instance, the self-defence or industry of the 

 animal. 



The food influencing most powerfully the parts of the 

 mouth, these must be considered as governing all the other 

 parts of the digestive system, and, consequently, they become 

 highly important for the purposes of classification, especially 

 as relates to genera and families. It may be added, that the 

 gradation to which the parts of the mouth are subjected, 

 proceeds nearly pari passu with that of the other organs to 

 which we can attach importance in classification, although 

 their reciprocal dependence is often very slight : such is the 

 gradation to which the wings are subject in insects. 



The skeleton of the Vertebrata being replaced in the Atmu- 

 losa and Annelida by the integuments, the masticatory organs 

 of the former have also disappeared, and are replaced by parts 

 belonging to the tegumentary system. 



In the genus Lumbricits, where the integuments are mem- 

 branaceous, the mouth is a simple orifice of the intestinal canal, 

 scarcely difl^ering from the anus ; hence these animals can 

 merely swallow their food without masticating it. In the leech 

 {Hirudo), which is higher in its organization than the earth- 

 worm, we find three fleshy jaws furnished with corneous teeth, 

 which form a sort of saw enabling them to cut the skin of 

 animals. This form of mouth, of which we find the first trace 

 in this genus, becomes more developed in the still more per- 

 fectly formed genus Eunice, where we find four pair of jaws, 

 of a different form and very strong, fixed in the mucous mem- 

 brane of the pharynx. In the Annulosa, as we have already 

 remarked, the trophi are but the anterior feet transformed, 

 and serving more directly for the pui'poses of digestion than 

 the others. The jaws analogous to those of Eiinice appear 

 to be wanting, but most probably they are represented by the 

 gastric teeth of Crustacea. In tracing the development of 



