Anatomy, Embryology, and Geology, when taken together, make it most probable that the Tree of the Mollusca is such as represented.
With the Mollusca we leave the Invertebrata, or animals without a backbone, and turn to the Vertebrata.
VERTEBRATA.
This division includes—1st, the Fishes; 2d, the Batrachia (frogs, etc.); 3d, the Reptiles (snakes, etc.); 4th, the Birds; 5th, the Mammals (animals suckling their young). They all possess a backbone, rudimentary in some fishes. This backbone is composed of separate bony pieces known as vertebrae; hence the name of Vertebrata given to the five classes just mentioned. Running through this backbone, spine, or vertebral column, as it is differently called, is seen the marrow or spinal cord,—a nervous cord which expands into the brain, which is inclosed by the skull-bones. Such a structure is never seen in a star-fish, insect, or mollusk. The Vertebrata never possess more than two pairs of limbs. The muscles moving these limbs are attached to bones, which, together with the skull and backbone, form the skeleton. The skeleton is the most characteristic feature of the Vertebrata, and nothing like it is met with in the Invertebrata, called also Evertebrata, that is, without vertebrae. There are apparent exceptions, such as the wing of an insect, among the Invertebrates, which is used like the wing of a bird; but the wing of the insect is only an expansion of the skin, whereas the wing of a bird is always supported by bone. The wing of the insect and that of the bird are said to be analogous, because they are used for the same purpose; they are not homologous, because they have not the same structure. The jaws of a Vertebrate are always parts of the head, never, as in many of the Crabs, modifications of the anterior limbs. There are found at different
