. Animal biology. Zoology; Biology. 464 GENERAL CONSIDERATIONS tion of the cells formed. Thus there may be found multinucleated cells, such as the striated muscular fibers. There are also living meshworks made up of cells which have only incompletely separated and remain in structural continuity. A tissue of the latter type is called a syncytium (Fig. 349), or, in the case of nervous tissue, a nerve net (Fig. 60). There are protoplasmic bridges which connect the epidermal cells of vertebrates, while in other cases fibrils extend from one cell into an adjacent cell and serve to coordinate the a
. Animal biology. Zoology; Biology. 464 GENERAL CONSIDERATIONS tion of the cells formed. Thus there may be found multinucleated cells, such as the striated muscular fibers. There are also living meshworks made up of cells which have only incompletely separated and remain in structural continuity. A tissue of the latter type is called a syncytium (Fig. 349), or, in the case of nervous tissue, a nerve net (Fig. 60). There are protoplasmic bridges which connect the epidermal cells of vertebrates, while in other cases fibrils extend from one cell into an adjacent cell and serve to coordinate the activities of the two. In no metazoan do the individual cells all live as they would if they were alone, but each is affected more or less by the proximity of the others. This is part of the organismal concept. Nevertheless, cells furnish convenient units on which to base our conceptions of morphology. 472. Organs and Systems.—In Chap. XXI the different systems were enumerated and brief references were made to some of the organs ?'^?^,...,^ fjt^''''"yw-. Fig. 349.—Semidiagrammatic representation of a syncytium, as seen in mesenchyme. included within each. As each phylum has been taken up in turn, many facts have been given in regard to the development of various systems in it. Here, however, the phylogenetic development of these different systems will be reviewed but only in general terms. It is desirable to begin the discussion of each system with those structures which in lower types, in which the system does not exist, perform the corresponding functions. 473. Tegumentary System.—In the lower protozoans, it has been seen that no cell wall, properly speaking, exists. Many of these forms, however, secrete shells of one kind or another which serve for protection. In the higher protozoans there is a surface layer, or pellicle, which is really a wall secreted by the cell for the purpose of protection. The epithelial cells of metazoans always have cell walls. In sponges
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