The Doctrine of Descent and Darwinism

Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: arranged radially round an axis, passing through the dorsal and ventral pole. The cavity, which in most other animals?for instance, in man?is termed the abdominal cavity, the space between the intestinal wall and the abdominal parietes, is deficient in them; but, on the other hand, from the stomach proceed...
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Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: arranged radially round an axis, passing through the dorsal and ventral pole. The cavity, which in most other animals?for instance, in man?is termed the abdominal cavity, the space between the intestinal wall and the abdominal parietes, is deficient in them; but, on the other hand, from the stomach proceed in general various kinds of tubes and Ijranchia, which to a certain extent replace the abdominal cavity. Fig. 2 represents a Medusa, Tiaropsis Diadema, after Agassiz. The darkly- shaded organs form the so-called ccelenteric apparatus. Of the Echinoderms, the reader is probably acquainted, at least with the star-fish (Asterias) and the sea-urchin (Echinus), of which the general form is likewise usually radiate. Besides a peculiar chalky deposit, or greater or less calcification of the skin covering, a system of water-canals forms a characteristic of this family. With these are connected the rows of suckers, which, by protrusion and retraction, serve as organs of locomotion. On account of the radiate, structure prevailing among the Echinoderms, Medusae, and Polypes, Cuvier believed them to be more nearly related, and introduced them altogether, under the name of Radiata. This similarity, however, is only superficial, for whilst, on the one hand, anatomy discloses the great difference of the Ccelenterata and Echinodermata, the history of evolution still more decidedly banishes the Echinoderm from this position, and connects them more closely with the next division. In this, that of the Vermes, the systematizer of the old school finds his real difficulty; in so many ways do they deviate from each other, so great is the distance between the lower and the higher forms; and after deducting the distinctive marks of orders, so little remains as a common character, so variegate...