12 



INTRODUCTORY LECTURE. 



the vertebral column, this structure being present in all the Enaima of 

 Aristotle, and absent in all his Anaima. Lamarck proposed, therefore, 

 the name of Vertehrata for the one class, and of Invertebrata for the 

 other. Now it will be observed that the Invertebrata are grouped 

 together by a negative character ; and I know not anj'^ instance where 

 such a chax-acter has been employed in zoology, in which very differ- 

 ently organised species have not been associated together. What in- 

 deed can be predicated in common of the snail, the bee, and the polype, 

 than that they are animals, and have no vertebral columns, and the 

 like negations. It was obvious also that there was no proportion or 

 equivalency between the Vertebrate and the Invertebrate groups, and the 

 idea of equivalency or proportion, as well as that of likeness, ought 

 always to govern the labours of the classifier. 



In the attempt to remedy this defect, the important discovery was 

 made that the vertebral column was subordinately related to a condition 

 of a much more important system in the animal body than the skeleton, 

 viz. the nervous system. Cuvier thereupon applied himself with inde- 

 fatigable industry to ascertain the arrangement of the nerves in the 

 Invertebrata, and after a long series of minute and elaborate dissections, 

 he discovered three modifications of that system, each of equal import- 

 ance with that which governed the vertebral character of the red- 

 blooded animals of Aristotle. Cuvier, accordingly, proposed to divide 



fi 



the animal kingdom into four primary groups or sub-king- 

 doms, viz. Vertebrata, Mollusca, Articulata, and Radiata. 



It is due to Hunter to state that the general results of 

 ills dissections of the nervous system are expressed in the 

 definitions of the same leading types as those of Cuvier; 

 but he made the minor diflerences which he had detected 

 in the Vertebrate series equal to those primary types of the 

 nervous system which now characterise the 3Iollusca and 

 Articulata of Cuvier, — a view which would have led to 

 erroneous results if ap^Dlied to the classification of the pri- 

 mary groups of animals. 



The sub-kingdom Vertebrata, or Myelencephala, is cha- 

 racterised by the disposition of the principal mass of the 

 nervous system in a median axis, consisting of the brain 

 and spinal chord {fig' !•), situated in the dorsal aspect of 

 the body, behind the heart and digestive system ; and in- 

 closed in a bony or cartilaginous case, constituting a verte- 

 bral column. The organs of the five senses, sight, hearing, 

 smell, taste, and touch, are almost always present. 



The respiratory organs communicate with the pharynx, or 

 anterior part of the alimentary canal. 



