Few departments of zoology have recently suffered more remarkable changes, both in classification and accepted views of structure, than the Polypi or Cœlenterata, and their immediate allies in the ascending scale, the Molluseoida,—greatly depending upon the more extended study of those animals of late years. We have been thus enabled to discover natural affinities which prima facie evidence would scarcely ever have indicated, as well as intrinsic differences which the same kind of evidence has hitherto been incapable of revealing to the mind.

Leading from the Protozoa to the Mollusca proper, the Cœlenterata and Molluscoida constitute an unbroken series of animals forming a considerable section of invertebrata, distinguished from the Protozoa by the development of true ova, and from the Mollusca by the property of gemmation developing compound examples of the principal types. Furthermore, the motion of the blood, or its equivalent, is effected either by ciliary action or by a propulsive organ; but when the latter occurs it is unfurnished with valves, so that the course of the circulation may be reversible in the same canals.