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19.1Animal Morphology and Classification

Figure 19-1 Basic structure of animals

The body of animals consists of three basic structures: the epidermis or skin, which covers the whole body; digestive tract, which absorbs nutrients from food; and muscular, connective and nerve tissues, which exist between the epidermis and digestive tracts.

The body of animals with diversified tissues and organs consists of three basic structures: the epidermis (skin), which covers the whole body; the digestive tract (stomach and intestines), which absorbs nutrients from food; and the muscular, connective, and nerve tissues, which exist between the epidermis and the digestive tract (Figure 19-1). During the early stages of development, the embryo is roughly divided into three layers: ectoderm, endoderm, and mesoderm. After the fertilized egg becomes the multicellular embryo through cleavage, the embryo forms a hollow ball enclosed by the epidermis, called blastula. Part of the blastula invaginates and ingresses to form the endoderm. This process is called gastrulation. The site from which invagination begins is called the blastopore. The portion remaining outside the invagination forms the ectoderm, which then fuses with the endoderm that has grown to the opposite side of the blastopore to form a tube-like structure. Cells that form between the ectoderm and endoderm or migrate from the outside to between these two layers form the mesoderm. Respiratory and digestive organs are formed from the endoderm, central nervous system and skin from the ectoderm, and muscle and connective tissues from the mesoderm. Animals in which the blastopore becomes the mouth are called protostomes, whereas those in which the blastopore becomes the anus or surrounding area are called deuterostomes. The body of the jellyfish, which belongs to the phylum Cnidaria, is made up only of tissues originating from the ectoderm and endoderm, and has no mesoderm. Sponges, which belong to the phylum Porifera, differ considerably from other animals in that they do not have any mesoderm tissues or digestive tract. Figure 19-2 shows the classification of animals existing today based on morphological comparisons.

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