CLASSIFICATION 
OF THE 
INVERTEBRATE ANIMALS. 
8 
§ 1. 
Tux 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. 
Crass I. Inrusorra. 
Cuass II. Rutzopopa. 
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. 
Crass III. Potyrr. 
Cuass IV. AcaLerua. 
Quass V. Ecninopermata, 
