Independently of the general agreement which necessitates the association of the Hydra, Actinia, and other Cœlenterate animals into one primary group of the animal kingdom, we must also expect a special morphological correspondence between the various forms of animals thus associated. In other words, a homological agreement ought to be determinable between the parts of animals included in any one subordinate section of the Cœlenterata with the parts of animals included in any other.

A comparison of the two primary sections of the Cœlenterata (Actinozoa and Hydrozoa), and of the various orders of these with one another, will show that such an agreement really exists, and that it is possible, by easily understood and thoroughly consistent modifications, to convert each special type into any of the others.