4:80 HISTORY OF PHYSIOLOGY. 



mals (1843). Mr. Owen observes that the arrangement of animals 

 into Vertebrate and Invertebrate which prevailed before Cuvier, was 

 necessarily bad, inasmuch as no negative character in Zoology gives 

 true natural groups. Hence the establishment of the sub-kingdoms, 

 Mollusca, Articulata, Radiata, as co-ordinate with Vertebrata, accord- 

 ing to the arrangement of the nervous system, was a most important 

 advance. But Mr. Owen has seen reason to separate the Radiata of 

 Cuvier into two divisions ; the Nematoneura, in which the nervous 

 system can be traced in a filamentary form (including Echinoderma, 

 Ciliobrachiata, Ccelelmintha, Rotifera,) and the Acrita or lowest divi- 

 sion of the animal kingdom, including Acalepha, Nudibrachiata, 

 Sterelmintha, Polygastria.? 



Sect. 3. Attempts to establish the Identity of the Types of Animal 



Forms. 



SUPPOSING this great step in Zoology, of which we have given an 

 account, the reduction of all animals to four types or plans, to be 

 quite secure, we are then led to ask whether any further advance is 

 possible ; whether several of these types can be referred to one com- 

 mon form by any wider effort of generalization. On this question there 

 has been a considerable difference of opinion. Geoffroy Saint-Hilaire, 11 

 who had previously endeavored to show that all vertebrate animals 

 were constructed so exactly upon the same plan as to preserve the 

 strictest analogy of parts in respect to their osteology, thought to 

 extend this unity of plan by demonstrating, that the hard parts of 

 crustaceans and insects are still only modifications of the skeleton of 

 higher animals, and that therefore the type of vertebrata must be made 

 to include them also : the segments of the articulata are held to be 

 strictly analogous to the vertebras of the higher animals, and thus the 

 former live within their vertebral column in the same manner as the 

 latter live without it. Attempts have even been made to reduce mol- 

 luscous and vertebrate animals to a community of type, as we shall 

 see shortly. 



Another application of the principle, according to which creatures 

 the most different are developments of the same original type, may be 

 discerned 12 in the doctrine, that the embryo of the higher forms of 

 animal life passes by gradations through those forms which are perma- 



11 Mr. Jenyns, Brit. Assoc. Rep. iv. 150. I2 Dr. Clark, Report, Ib. iv. 113 



