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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: CHAPTER III. v THE ORGAN OF VISION. The perception of light is not equally valuable to all classes of animals. It seems to be specially connected with the power of locomotion. Sessile or sedentary animals, as a rule, do not possess any form of visual organ; while very free and active animals, even of lower organisation, have well-marked eyes. The Echinodermata, for example, are far more highly evolved creatures than the Medusas, but their habits are comparatively sluggish, while the Medusae lead a wandering, predatory life; and we find that the former class are apparently quite eyeless, while the latter have distinct ocelli, which in some cases reach a considerable complexity of structure. Still more clearly is this connection made evident by the metamorphoses of many creatures which pass from a free to a fixed state. The young barnacles and balani are active, locomotive animals, furnished with eyes, antennae, and limbs; but after a period of activity, they finally fix themselves upon some solid object, and undergo a loss of all their higher sense-organs. Similar changes take place among the parasitic Entomostraca, the Tubicolar Annelids, and many Mollusca. These must be regarded as cases of degradation or retrogressive development. Conversely, the Medusae are shown, by their peculiar mode of development, to be the descendants of hydraform polypes. During their sessile stage, when they exactly resemble the true Hydroida, they are as destitute of eyes as the other members of that order. But when theyacquire their tentacles and assume the free mode of life, the ocelli are produced together with the other mature organs. This must be regarded as a case of progressive development. If we examine the various classes of animals in order, the same general connection between ...
| ISBN-13 | 9780217755986 |
|---|---|
| ISBN-10 | 0217755984 |
| Weight | 0.60 Pounds |
| Dimensions | 9.00 x 6.00 x 0.41 In |
| List Price | $19.99 |
| Format | Paperback |
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| Pages | 178 pages |
| Publisher | |
| Published On | 2009-08-01 |
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