INTRODUCTION. 



23 



NAUTILUS WITH THE SHELL : DIVISION MOLLUSCA. 



originating from the lower part of the brain, passing through a large hole in the base of the skull 

 and running down through a bony canal, formed by the vertebral column, of which the skull is, in 

 fact, only the anterior portion. As this set of organs — the brain and the spinal cord, the skull and 

 the vertebral column — while possessed by no other animals, is, with a few rare exceptions, con- 

 stantly present in these, its existence will always serve to distinguish them from the rest of the 

 animal kingdom. They are accordingly called vertebrate animals, and the division which they 

 form, Vertebrata. 



These, however, are not the only characters possessed in common by vertebrate animals. The 

 vertebral column forms only a portion of an internal bony framework or skeleton, which serves 

 for the support of the soft portions of the body, and by furnishing the necessary points of attach- 

 ment for the muscles, assists in effecting the movements of the animal. This framework generally 

 consists of the vertebral column, including the skull ; the jaws, which are regarded as appendages 

 of the vertebra?, of which the skull is considered to be composed ; the ribs, a series of bony arches, 

 articulated at one extremity with the bones of the vertebral column, and at the other either at- 

 tached to a central bony piece, the sternum, or lying perfectly free in the tissues of the body ; anil 

 the limbs, which are never more than four in number. The jaws in these animals always separate 

 in a vertical direction, so that the opening of the mouth is transverse. They all have red blood and a 

 muscular heart. Their reproduction is sexual, and the sexes are never united in the same individual. 



The animals constructed upon this type are the most highly organized of. living beings. In no 

 others is the nervous matter, the seat of sensation, intelligence, and volition, presented in so con- 

 centrated a form ; in none are the senses so perfect, or the various functions of the animal econo- 

 my so completely isolated. 



We thus see that animals are constructed upon Five Primary Types or Plans, of which all the 

 varied forms presented by these creatures are but modifications ; as though the Creator, in design- 



