Elementary text-book of zoology, tr Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote elementarytextbo01clau Year: 1892-1893 34 GENERAL PART. atozoon is hat-shaped; while in Crustacea it has the form of a cell, with long radiating processes (fig. 20). Epithelial tissues consist of aggregations of cells which as simple or stratified layers cover the external and internal surfaces of the body, and line its closed spaces (endotheUum). According to tke different shape of the cells composing it, we distinguish cylin- drical, ciliated, and pavemen
Elementary text-book of zoology, tr Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote elementarytextbo01clau Year: 1892-1893 34 GENERAL PART. atozoon is hat-shaped; while in Crustacea it has the form of a cell, with long radiating processes (fig. 20). Epithelial tissues consist of aggregations of cells which as simple or stratified layers cover the external and internal surfaces of the body, and line its closed spaces (endotheUum). According to tke different shape of the cells composing it, we distinguish cylin- drical, ciliated, and pavement epithelium. In the first case the cell?, in consequence of the elongation of the long axis, are cylindrical (fig. 21, c); in the second, the free surface of the cells is beset with vibratile cilia or flagella (fig. 21, d), which are continuous with the living protoplasm of the cell. If only one flagellum projects from the cell (sometimes a flat cell fig. 21, 6) then the name flagellate cell is applied (collared cell of sponges, fig. 21, e). Finally, in the case of pavement epithelium (fig. 21, a) the cells are flattened; and if there FIG. 21.—Various kinds of epithelial cells, a, Flat cells, b, flat cells with fiagella (from a Medusa), e, cylindrical cells, d, ciliated cell, e, flagellate cell with collar (from sponge). /, cylindrical cell with porous border (intestinal epithelium). is more than one layer the superficial cells are flat, while those in the deeper layers are more and more rounded. While the cells of the lower layers retain their semi-fluid character, and are occupied in continual cell division and growth; those of the upper layers possess a firmer consistency, gradually become horny, and are thrown off as scales or continuous flakes, to be replaced by the continuous growth of the lower layers. Thick stratified layers of cornified cells, almost fused with one another, give rise to indurated or horny structures (nails, claws, hoofs), which may form a more or
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