18 COMPARATIVE DENTAL. ANATOMY 



Vertebrates have a cerebro-spinal axis and a 

 strong bony column composed of separate pieces 

 called vertebrae, which are connected together by 

 ligaments and are more or less movable. The 

 sub-kingdom of the Invertebrates comprises all 

 classes of animals which do not have a vertebral 

 column, whatever their various plans of struc- 

 ture may be. The class of the Vertebrates is 

 therefore homogeneous, and that of the Inverte- 

 brates very heterogeneous. 



The sub-kingdom of the Invertebrates includes 

 all animals which have no internal backbone or 

 vertebral column; such as the Infusoria, Hy- 

 droids, Eadiata, Worms, Insecta, Crustacea, Mol- 

 lusca, etc. 



There is no spinal cord with its anterior en- 

 largement, the brain in Invertebrates, but instead 

 the nervous system consists of chains of ganglia 

 scattered throughout the system, arranged in 

 rows or circles connected by cords of nervous 

 substance and giving filaments to various parts 

 of the organism. 



The digestive system is simple. The stomach 

 may be a single sac with but one opening as in 

 the Hydroids, or a complete alimentary canal with 

 two openings, — the oral and anal, — as in the 

 worms, insects, mollusks, etc. In the lower forms 

 specialized digestive glands do not appear, but 

 are present in the higher orders. 



