HOMOLOGIES IN DISCONNECTED ANIMALS. 27 



and not upon affinity. While, for instance, the head of 

 fishes exhibits the most striking homology with that of 

 reptiles, birds, and mammalia, as a whole, as well as in all 

 its parts, that of Articulata is only analogous to it and to 

 its part. What is commonly called the head in Insects is 

 not a head like that of the Vertebrata : it has not a distinct 

 cavity for the brain, separated from that which commu- 

 nicates below the neck with the chest and abdomen ; its 

 solid envelope does not consist of parts of an internal 

 skeleton surrounded by flesh, but is formed of external 

 rings, like those of the body, soldered together ; it con- 

 tains but one cavity, which includes the cephalic gan- 

 glion, as well as the organs of the mouth and all the 

 muscles of the head. The same may be said of the chest, 

 the legs and wings, the abdomen, and all the parts they 

 contain. The cephalic ganglion is not homologous to the 

 brain, nor are the organs of the senses homologous to 

 those of Vertebrata, even though they perform the same 

 functions. The alimentary canal is formed in a very dif- 

 ferent way in the embryos of the two types, as are also 

 their respiratory organs ; and it is as unnatural to identify 

 them, as it would be still to consider gills and lungs as 

 homologous among Vertebrata, now that Embryology has 

 taught us, that in different stages of growth, these two 

 kinds of respiratory organs exist in all Vertebrata in very 

 different organic connections one from the other. 



What is true of the branch of Articulata when com- 

 pared to that of Vertebrata is equally true of the Mol- 

 lusks and Eadiata when compared with one another or 

 with the two other types, as might easily be shown by a 

 fuller illustration of the correspondence of their structure 

 within these limits. Tin's inequality in the fundamental 

 character of the structure of the four branches of the 



