12 



INTRODCCTOKY LECTURE. 



viz. the nervous system. Cuvier* thereupon applied himself with 

 indefatigable 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 importance with that which governed the vertebral character 

 of the red-blooded animals of Aristotle. Cuvier, accordingly, pro- 

 posed to divide the animal kingdom into four primary groups or 

 sub-kingdoms, viz. Vertebrata, Mollusca, Articulata, and 

 Radiata.^ 



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

 his dissections of the nervous system are expressed in the 

 definitions of the same leading types as those of Cuvier > 

 but he made the minor differences which he had detected 

 in the Vertebrate series equal to those primary types of the 

 nervous system which now characterise the Mollusca and 

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

 erroneous results if applied 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 {Jly. 1.), situated along the dorsal aspect 

 of the body, behind the heart and digestive system ; and 

 inclosed 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. 

 The mouth is provided with two jaws, placed one above or in front 

 of the other. 



The blood is red. 

 2 .\ A// / \|j / The heart is a compact muscular organ, 

 having never fewer than two cavities, an 

 auricle and ventricle, propelling the blood 

 through a closed system of arteries and veins. 

 The muscles surround the bony or gristly 

 levers on which they act. 



The locomotive memb«rs never exceed two 

 pairs. 



The sexes are distinct and no species is 

 parthenogenetic. 



In the sub-kingdom Mollusca, or HeterO' 



xir. 



t XV. 



X X. Vol. iii, p. 10. 



