224 ESSAY ON CLASSIFICATION. Part I. 



of blood are also in arches, -which dO' not coincide with the medial line of the 

 body. The nervous system consists of diffused ganglia, united by threads, the 

 larger ones being around the oesophagus. The nervous system and the organs 

 of sense appear late ; the motions are slow and powerless. 



IV. Tlie Veiiebrate Tijpe. This is, as it were, composed of the preceding 

 types, as we distinguish an animal and a vegetative system of the body, which, 

 though influencing one another in their development, have singly a jjeculiar typical 

 organization. In the animal system, the articulation reminds us of the second 

 type, and the discharging and receiving organs are also placed at opposite ends. 

 There is, however, a marked difference between the Articulates and the Vertebrates, 

 for the animal system of the Vertebrates is not only doubled along the two sides, 

 but at the same time upwards and downwards, in such a way that the two lateral 

 Avails which unite below circumscribe the vegetative system, Avhile the two tending 

 upward surround a central organ of the animal life, the brain and spinal marrow, 

 which is wanting in Invertebrates. The solid frame represents this type most com- 

 pletely, as from its medial axis, the backbone, there arise upward arches which close 

 in an upper crest, and downward arches which unite, more or less, in a lower crest. 

 Corresponding to this we see four rows of nervous threads along the spinal marrow, 

 which itself contains four strings, and a quadripartite grey mass. The muscles 

 of the trunk form also four principal masses, which are particularly distinct in the 

 Fishes. The animal system is therefore doubly symmetrical in its arrangement. It 

 might easily be shown how the vegetative systems of the body correspond to the 

 type of Mollusks, though influenced by the animal system. 



From the illustrations accompanying this discussion of the great types or branches 

 of the animal kingdom, and still more from the paper published by K. E. von 

 Baer in the Nova Acta,^ it is evident, that he perceived more clearly and earlier 

 than any other naturalist, the true relations of the lowest animals to their respective 

 branches. Pie includes neither Bryozoa nor Intestinal Worms among Radiata, as 

 Cuvier, and after him so many modern writers, did, but correctly refers the former 

 to the Mollusks and the latter to the Articulates. 



Comparing these four types with the embryonic development, von Baer shows 

 that there is only a general similarity between the lower animals and the embryonic 

 stages of the higher ones, arising mainly from the absence of differentiation in the 

 body, and not from a typical resemblance. The embryo does not pass from one 

 type to the other; on the contrary, the type of each animal is defined from the 



■^ Beitrage zur Kenntniss tier niedern Thiere, animals. These " Beitriige," and the papers in which 



Nova Acta Academise Natui-a; Curiosorum, vol. 13, Cuvier clianK-terized for the first time the four great 



Part 2, 1827, containing seven papers, upon Aspido- types of the animal kingdom, are among the most 



gaster, Disfoma, and others, Cercaria, Nitzschia, Poly- important contributions to general Zoology ever 



stoma, Planaria, and the general affinities of all published. 



