The oyster is an animal which belongs to a much lower class of mollusca--namely, to the class called _Lamellibranchiata_, from the plate-like (or lamellar) structure of the gill. To that class also belongs the scallop (_Pecten_), the mussel (_Magilus_), the fresh-water mussel (_Anodon_), the razor-shell (_Solen_), the cockle (_Cardium_), species with a long fleshy tube such as _Mya_, stone-perforating shells such as _Pholas_, and the well-known wood-boring "ship-worm" (_Teredo_)--which is no "worm" at all--with a multitude of other forms. Certain other animals (which, like the Lamellibranchs, all have a shell divided into two valves) form another still lower class called _Brachiopoda_, a class which we may, at least provisionally, consider as belonging to the mollusca. These _Brachiopods_ are also called "Lamp-shells," from a certain resemblance which many of them show to the form of a classical lamp. They are interesting, because in very ancient times they seem to have held that place in the world's animal population which is now held by the Lamellibranchs, by which, as they died out, they have been gradually replaced till but comparatively few forms survive. Some of these, however, are of great antiquity, and one of them, _Lingula_, is, though still living, one of the most ancient of all known animals. We may next pass to a small sub-kingdom which includes the curious and inert animals before referred to[16] as "Sea-squirts," Tunicaries or Ascidians, and which constitute the sub-kingdom TUNICATA. These are marine organisms of very simple but very peculiar structure which sometimes grow up in compound aggregations. Certain forms (_e.g._, _Pyrosoma_) are luminous at night and may be seen swimming about in the ocean like so many red-hot urn-heaters. As we shall hereafter see, the reproductive processes and the earlier stages of existence of these creatures possess much interest, and have afforded strong grounds for regarding them, in spite of their lowly organization, as very close allies of the highest animals or _Vertebrata_. Returning now to the "lobster" (lately mentioned as one of those animals commonly called "shell-fish") we may regard it as an example of what is by far the most numerous of all the sub-kingdoms of animals. This sub-kingdom is made up of animals with jointed feet or "Arthropods," and the ARTHROPODA are subdivided into four classes--1, _Crustacea_; 2, _Myriapoda_; 3, _Arachnida_; and 4, _Insecta_; and it is to the first of these four classes that the lobster belongs. The class _Crustacea_ contains, besides the lobster (and its near allies, hermit-crabs, prawns, shrimps, and cray-fish), all crabs, including those very quaint-looking animals (now so often seen in our living collections), the king-crabs (_Limulus_), and a variety of more or less strangely different forms such as the following:--