Elementary text-book of zoology, general Elementary text-book of zoology, general part and special part: protozoa to insecta elementarytextbo00clau Year: 1892 28 OBOANIZATIOX AXD DETELOPME^?.^ OF AXIMALS IX QENEEAL. which are placed one behind the other, and more or less completely resemble each other in structure {Annelids, fig. 15). The si;ccessive segments may in structure and function appear completely equiva- lent, and represent, like the antimeres of the Radiata, individuals of a lower order, which on the severance of their mutual connec- tion can acquire independence and remain alive f
Elementary text-book of zoology, general Elementary text-book of zoology, general part and special part: protozoa to insecta elementarytextbo00clau Year: 1892 28 OBOANIZATIOX AXD DETELOPME^?.^ OF AXIMALS IX QENEEAL. which are placed one behind the other, and more or less completely resemble each other in structure {Annelids, fig. 15). The si;ccessive segments may in structure and function appear completely equiva- lent, and represent, like the antimeres of the Radiata, individuals of a lower order, which on the severance of their mutual connec- tion can acquire independence and remain alive for a shorter or longer period {proglottis of Cestodcs). In animals of higher organization the segments are much more intimately connected, and are mutually dependent, but they lose at the same time their complete homonomy. In the same degree as the metameres acquire an unlike structure, and corresponding to this a varying importance in the life of the oi'gan- ism, they lose their individual independence, and sink more and more to the value of organs. The metameres in the polymorphous colonies are quite analogous to the segments of the individual. In them there follow, one behind the other, similar groups of different individuals, each of which fulfils singly the conditions necessary for existence, and there- fore can continue to live as a colony of a lower order when separated from the stock {Eudoxia, Diphyes, fig. 16). The distinction into a higher and lower order also holds for organs. There are organs which are reducible to a single cell, or to an aggregation of equivalent cells (simple organs), and others in the formation of which various cells and tissues (compound organs) partici- pate, and which frequently, in their turn, may be divided into parts different in structure and function. The compound organs of higher order are composed of different parts which function as organs of a lower order. These, again, are composed of various kinds of cells and cell derivates, which
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