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CLASSIFICATION 



or THE 



INVERTEBRATE ANIMALS. 



§1- 



The invertebrate animals are organized after various types, the limits 

 of which are not always clearly defined. There is, therefore, a greater 

 number of classes among them than among the vertebrates. But, as the 

 details of their organization are yet but imperfectly known, they have not 

 been satisfactorily classified in a natural manner. 



There are among them many intermediate forms, which make it difficult 

 to decide upon the exact limits of various groups. 



The following division, however, from the lowest to the highest forms of 

 organization, appears at present the best : 



ANIMALIA EVERTEBRATA. 



INVERTEBRATE ANIMALS. 

 Brain, spinal cord, and vertebral column, absent. 



FIRST GROUP. 



PROTOZOA. 



Animals in which the different systems of organs are not distinctly sep- 

 arated, and whose irregular form and simple organization is reducible to 

 the type of a cell. 



Class I. Infusoria. 

 Class II. Rhizopoda. 



SECOND GROUP. 



ZOOPHYTA. 



Animals of regular form, and whose organs are arranged in a ray-like 

 manner around a centre, or a longitudinal axis ; the central masses of the 

 nervous system forming a ring, which encircles the oesophagus. 



Class III. Polypi. 

 Class IV. Acalephä. 

 Class V. Echinodkrmata, 



