. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. SYMMETRY. 267 as Eadiata is inadmissible, and so much the more so since the radial arrangement of the structure exhibits some transitions towards a bilateral symmetry. The Echinodermata are separated from the Cielenterata by the possession of a separate alimentary canal and vascular system, and also by a number of peculiar features both of organization and of development. The arrangement of the parts round the axis of the body is usually pentamerous. Nevertheless when the rays are more nume
. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. SYMMETRY. 267 as Eadiata is inadmissible, and so much the more so since the radial arrangement of the structure exhibits some transitions towards a bilateral symmetry. The Echinodermata are separated from the Cielenterata by the possession of a separate alimentary canal and vascular system, and also by a number of peculiar features both of organization and of development. The arrangement of the parts round the axis of the body is usually pentamerous. Nevertheless when the rays are more numerous, irre- gularities in the repetition of the similar organs are met with. If we take as the fundamental form of the Echinoderm type a spheroid with the principal axis somewhat shortened and the poles flattened and dissimilar, the long axis of the radial body will be this chief axis, and the mouth and anus the two poles (oral and anal poles). We can imagine five planes pass- ing through the long axis of this spheriod, each of which will divide the body into two symmetrical halves. The perfect correspondence of these halves is, in the body of Echinoderms, disturbed by the dif- ferent forms and significance of the two poles, so that our representation is not an exact one. The ten meri- dians, which are separated from one another by equal intervals and fall in these five planes, are differently related to one another, inasmuch as five alternate ones, which are called the chief rays, or radii, contain the most important organs, the nerves, the vascular trunks, the ambu- lacral feet, etc., while the other five meridians constitute the intermediate rays or inter-radii, and also contain certain organs (fig. 206). It is only in cases of complete equivalence of the radii and inter-radii that the echinoderm body presents a pentamerous radial arrangement (regular Echinoderms). It is, however, easy to show that this regular radial symmetry never occurs in its perfect form. Since one orga
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