5. The Gasteropoda are extremely numerous; most of them have univalve shells, but many of them have none, as the Garden Slug (Limax, fig. 35). The Whelk (Buccinum undatum), Perriwinkle (Littorina littoria), Garden Snail (Helix aspersa, fig. 35), and the Wentle-trap (Scalaria, fig. 36), are the most familiar examples of this class. These Mollusca walk by means of the mantle, which is muscular, and capable of alternate contraction and expansion; they breathe by means of lungs on their back beneath the shell, and to which there is an opening in their side. What are usually called the horns are four in number, two short and two long; they are tentaculæ, but what is peculiar in them is the circumstance of having the eyes placed at their ends. These Mollusca have a sort of valve, which, when they retire into the shell, closes it like a lid; it is called the "Operculum;" in some cases it is horny, as in the Perriwinkle, and in others, resembles shell; the shells of these mollusca are coiled into a spiral, this is caused by the shell always growing by additions to the edges of the mouth, and in nearly every case this spiral turns in the same direction that a screw does, but in a very few it turns in the opposite direction, as in the "Fusus contrarius." 6. The Cephalopoda include the "Cuttle-Fish," and Nautilus (fig. 33). They are furnished with eight or ten tentacula or arms, which spring from the head, and which are covered with suckers; the mouth is in the centre, and these tentacula are used to lay hold of their prey and convey it to the mouth; they have two perfect eyes, and they breathe by gills. The Nautilus has a univalve shell, which is of a very graceful and elegant form; its interior is divided into chambers, with a syphon running through them, by which the air is exhausted or compressed so as to cause them to sink or swim; the animal occupies only the outer chamber; the extinct Ammonite belonged to this class. It is from the Cuttle-Fish (Sepiæ) that the beautiful dark brown pigment, known as "sepia," is obtained, and also the material of which Indian-ink is made. THE VERTEBRATA. The Vertebrate animals are those which have a brain and spinal cord inclosed within a bony case, an internal skeleton, to which are affixed the muscles, and which is moved by them; they all possess four extremities, more or less modified in form, with the exception of the Serpents. They are divided into four classes, Fishes, Reptiles, Birds, and Mammals. FISHES.