. Elements of biology, with special reference to their rôle in the lives of animals. Biology; Zoology. A PPENDA GE ^^.. Fig. 84.—Nereis, the com- mon sandworm of the sea shore. In the living animal four eyespots may be dis- tinguished in the dorsal sur- face of the head. The broad appendages are composed of two lobes. The upper lobes, visible in the figure, contain many blood vessels and serve as respiratory organs. The lower lobes are tra- versed by stiff setae and serve motion. No such ideal arrangement is found, for in all known metameric ani- mals various segments are modified in various w
. Elements of biology, with special reference to their rôle in the lives of animals. Biology; Zoology. A PPENDA GE ^^.. Fig. 84.—Nereis, the com- mon sandworm of the sea shore. In the living animal four eyespots may be dis- tinguished in the dorsal sur- face of the head. The broad appendages are composed of two lobes. The upper lobes, visible in the figure, contain many blood vessels and serve as respiratory organs. The lower lobes are tra- versed by stiff setae and serve motion. No such ideal arrangement is found, for in all known metameric ani- mals various segments are modified in various ways to serve special functions. For example, in the earthworm several of the most anterior segments that appear during the development of the animal fuse to- gether to form the leading and correlating element, the head. This fusion of anterior segments into a single unit, the head, is termed cephalization. The number of seg- ments involved in forming the head varies in different varieties of annelids and among other segmental forms. The num- ber of segments that have gone to form the human head is a matter of dispute, for. the process of cephalization in this highly modified form is obscure, but indications are that the process has taken place. The Principle of Homology. In some annelids, for example the sandworm, Nereis (Fig. 84), each segment, except the most anterior, is equipped with a pair of lateral appendages. Each appendage is comparable with the others, being derived in the same way from the same materials during development. Such structures that have the same origin are termed homolo- gous structures. Thus the ca?lom is homol- ogous throughout the series of forms in which it is developed; the wing of a bird and the fore limb of a cat, being as organs of Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble t
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