396 Professor Agassiz on the Development of Oi'ganhed Beincji 



mammifera, and that these last approach verj' nearly to man 

 in their organization ; so that, regarded as a whole, these four 

 classes seem to be the successive degrees in the manifesta- 

 tion of the type of the vertebrata. 



Invertebrate animals do not appear to be subjected to the 

 same laws of developement as vertebrate animals. For, what 

 superiority can we assign to the Vermes, which form a part of 

 the Articulata, over the Cephalopods, which belong to the 

 Mollusca \ and on what grounds can we place the Acephala 

 above the Echinodermata, which are nevertheless true Radiata I 

 The truth is, that the existence of the Invertebrata cannot be 

 referred to the same principle which is manifested in the de- 

 velopment of the Vertebrata, which latter are undoubtedly 

 linked with the existence of man. Ascending to the epoch of 

 the first appearance of fishes, we find that the Radiata, the 

 Mollusca, and the Articulata, have pursued a series of meta- 

 morphoses, which has not at all elevated them to higher types. 

 The corals of the most ancient formations are analogous to 

 those of our seas. The Echinodermata go back equally far ; 

 and if we notice the important modifications in their relations 

 with the surface, and in the distribution of their families in 

 the different geological epochs, we find no indication of their 

 genetic connection with the other classes. The same is the 

 case in the three classes of Mollusca ; the Acephala of the 

 early periods are, it is true, less free, their symmetry is not 

 marked in so decided a manner on the sides of the longitudi- 

 nal axis of the animal, the anterior and posterior regions of 

 the body are not so clearly defined, the variety of species, 

 genera, and families, is less considerable than in more recent 

 epochs ; but, notwithstanding all that, they advance in a 

 parallel line with the Gasteropods and the Cephalopods, which 

 have at no period been subjected to a greater amount of mo- 

 difications. With regard to the Articulata we may make the 

 same observation, notwithstanding the imperfection of our in- 

 formation respecting the fossil species of that division. The 

 Crustacea, which are placed at the head, have not by any 

 means been preceded by the insects and the Vermes ; any more 

 than the Cephalopods have been by the Gasteropods and the 

 Acephala, and the Echinodermata by the Medusie and Polypi. 



