It would be foreign to the object of this work to enter into minute division of the innumerable creatures which swarm on the ocean and on its confines. We shall perhaps best consult the convenience of our readers by adopting the following simple arrangement of these animals into I. PROTOZOA, including the _Spongiadæ_, _Infusoria_, and _Foraminifera_. II. POLYPIFERA, including the _Hydræ_, _Sertularia_, and _Pennatulariæ_. III. ECHINODERMATA, or Sea-urchins and Star-fishes. Our space will prevent our doing more than presenting to the reader in succession the most characteristic types of each of these groups. I. THE PROTOZOA. The Protozoares represent animal life reduced to its most simple expression. They are organized atoms, mere animated and moving points, living sparks. As they are the simplest forms of animal life as regards their structure, so also they are the smallest. Their microscopic dimensions hide them from our view. The discovery of the microscope was a necessary step to our becoming acquainted with these beings, whose existence was ignored by the ancient world, and only revealed in the seventeenth century by the discovery of the microscope. When armed with this marvellous instrument, applied to examine the various liquid mediums--as when Leuwenhoek, for example, applied the magnifying glass to the inspection of stagnant water, with its infusions of macerated vegetable and animal substances--when he scrutinized a drop of water borrowed from the ocean, from rivers, or from lakes, he discovered there a new world--a world which will be unveiled in these pages. Some modern writers believe that the Protozoa is a mere _cellular organism_, that being the principle and end of organization, such as we find it in the cellular vegetable. According to this hypothesis, the Protozoares would be the cellulars of the animal kingdom, as the Algæ and Mushrooms are of the vegetable world. This idea is so far wrong, that it has been founded upon the empire of pure theory. "In reality," says Paul Gervais and Van Beneden, "the animals to which we extend it very rarely resemble elementary cellulars." The tissue of which the bodies of the Protozoa are composed is habitually destitute of cellular structure. They are formed of a sort of animated jelly, amorphous and diaphanous, and have received from Dujardin the name of _Sarcoda_, or soft-fleshed animals. Infinitely varied in their form, the Protozoares are furnished with _vibratile cilia_, which are organs of locomotion belonging to the lower animals inhabiting the liquid element. Their bodies are sometimes naked, sometimes covered with a siliceous, chalky, or membranous cuirass. They are divided into two great classes, the _Rhizopoda_ and _Infusoria_. SPONGIA.