In this communication an Actinozoon (Actinia) was compared with a Hydrozoon (Hydra), and the various Sub-orders of the Hydrozoa were compared with one another.

The author agreed with Agassiz in regarding the radiating chambers of an Actinia as the homologues of the radiating canals of a medusa, but he differed from him as to the true homologies of the differentiated stomach-sac of Actinia; for while Agassiz regards this as represented by the proboscis or hypostome of the Hydra inverted into its body cavity, Professor Allman maintains that it is impossible on this supposition to conceive of the structure of Actinia; and on comparing a Hydra with an Actinia, he imagines the tentacle to become connate for a greater or less extent with the sides of the hypostome and with one another, so that the hypostome of the hydra, while retaining its normal position, will thus become the stomach of the Actinia, and will at the same time become connected with the outer walls by a series of radiating lamellæ—the connate tentacle walls—separated from one another by radiating chambers, the cavities of the tentacles; while such portions of the tentacles of Hydra as still continue free will be represented by a single circle of the tentacles of Actinia.