Characters of Articulates, 160; Of Worms and Mollusks, 161: Type Characters, 161; Typical Animals, 162; Systematic Tables and Classification, 163. External Characters of Salamanders, 163, 164; Comparison with Cat, 165; Differ- ences between the two, 166; Young Salamander, 166; Skeleton of Cat, 167; Vertebral Column and Spinal Cord, 169, 170; Bones of the Ankle and Wrist, Importance of studying Young Forms, 174; Wing Bones of an Adult Bird, 175; Embryo Birds, and Wing Bones of same, 176; Leg Bones of an Adult Bird, 177; Leg Bones of Embryo Bird, 178; Leg Bones of Young Toad, 180. Sub-Kingdom Mollusca, Class Gasteropoda, 181; Class Acephala, 182; Sub-King- dom Arthropoda, Class Insecta, 183; Class Myriopoda, 184; Class Arachnida, 195; Sub-Kingdom Vermes, Class Annelida, 185; Class Crustacea, 186; Sub- FIRST BOOK OF ZOOLOGY. CHAPTER I. FRESH-WATER SHELLS. FOR these lessons, it has been deemed best to commence with the shells of mollusks, such as snail-shells and mussels. They are better objects to examine than insects, being more simple in structure, and less liable to be broken in handling. When found alive, their habits can be readily studied, as they can easily be kept alive in jars filled with water. 1. Let the pupils first make a collection along the shores of some lake or river, picking up all the different kinds of shells they meet with. The waves will have thrown them up on the shores, or in times of drought the waters will have left them exposed. Certain kinds are very small, though they will be found by sharp looking. Most of the shells collected will be empty, and these shells are called dead shells, because the soft-bodied creatures once contained in them have died and decayed, leaving the hard, limy shells. Some of the shells collected may contain the animal, and at one time each of them possessed a little crea ture within, which was the fabricator of the shell. 2. Remember that the shell is not a house built by the snail, as a wasp builds its nest, but the shell is a part of the animal, and is connected to it by certain muscles, so that it cannot leave the shell, as many suppose. The empty, or dead, shells, are to be studied first. Looking over the shells collected, we shall find some of the following kinds : FIG. 1.-FRESH-WATER Shells. A number of fresh-water mussel-shells, also, will probably be collected. FIG. 2.-FRESH-WATER MUSSEL-SHELL. These are to be reserved for future examination. Those having a spiral turn or twist are called snail shells, and are to be studied first. 3. Let the pupils pick out from their collections the shells like these: FIG. 3.-FRESH-WATER SNAIL-SHELLS. The different spiral turns, or twists, are called whorls, and the whorls together form the spire. The opening into the shell is called the aperture, and the line separating the whorls is called the suture. The pointed end of the spire is called the apex. In some shells the spire is elongated; in others the spire is short; in others still the spire is depressed or flattened. FIG. 5. SPIRE ELONGATED. SHORT. FLATTENED. 4. If the shell is held in the hand, with the aperture toward the holder, and the spire pointing upward, as in the figures drawn, the aperture will be either toward the right hand, or toward the left hand. In the figures already given, the aperture is on the right hand, and these shells are called dextral, or right-handed, shells. Shells having the aperture on the left hand when held in the way above described, are called sinistral, or left-handed, shells. Let the pupils here examine all the shells they have collected, holding each one with the spire pointing upward, and the aperture toward them, and separate the dextral shells from the sinistral shells. As sinistral shells are not so common as the other kind, it may be that none will be found in the first collections made by the pupils. The following is a figure of a sinistral shell: FIG. 6. SINISTRAL SHELL. |