98 Mr. J. D. Dana on Zoophytes. XII. — On the Geographical Distribution and Classification of Zoophytes*. By James D. Dana. Heat, light, pressure, and means of subsistence, influence more or less the distribution of all animals ; and to these causes should be added, for water species, the nature or condition of the water, whether fresh or marine, pure or impure, still or agitated. Next to the character of the water, heat is the most prominent limit-ing agent for marine animals, especially as regards latitudinal extent, while light and hydraulic pressure have much influence in determining their limits in depth. Although these causes fix bounds to species and families, they do not necessarily confine tribes of species to as small limits. This is sometimes the case, and it is nearly true of a large group of zoophytes; yet other tribes and orders include species whose united range comprises all the zones, from the equator to the polar ices, and every depth, to the lowest which man has explored, affording traces of life. Order Hydroidea. — The Hydroidea are met with in all seas and at great depths, as well as at the surface. The tropics and the cold waters of the frigid zone have their peculiar species, and a few are found in fresh waters. The rocks and common marine plants of the sea-coast, the dead or living shell, or the floating fucus of the ocean, are often covered with these feathery corals ; and, about reefs, they occasionally implant themselves upon the dead zoophyte, forming a mossy covering, taking the place of the faded coral blossom. The species are most abundant, however, in the waters of the temperate zone, and are common upon some portions of our own coast. Order Actinoidea. — The Actinoidea are marine zoophytes. All oceans have their species, yet in the torrid zone they more especially abound, and display most variedly their colours and singular forms. The soft Actinidce and the Alcyonaria have the widest range, occurring both among the coral reefs of the equatorial regions, and, to the north and south, beyond the temperate zone. The Mediterranean affords species of Gorgonia, Corallium and Alcy-onium, besides numerous Actinia. The coasts of Britain have also their Alcyonia and Actinia, and from far in the northern seas, come the Umbellularia, and some other species of the Pen-natula family. Among the coral-making Actinaria, the Madrepore and Astraea tribes are almost exclusively confined to the coral-reef seas, — a * From Silliman's American Journal for March and Mav 1847.
BioStor
