File:The royal natural history (1893) (14778230041).jpg

Identifier: royalnaturalhist612lyde (find matches)
Title: The royal natural history
Year: 1893 (1890s)
Authors: Lydekker, Richard, 1849-1915 Sclater, Philip Lutley, 1829-1913 Frostick, W. B., former owner. DSI Brooks, W. T., former owner. DSI
Subjects: Zoology Natural history
Publisher: London and New York : Frederick Warne & Co.
Contributing Library: Smithsonian Libraries
Digitizing Sponsor: Smithsonian Libraries

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-fish. A crinoid (illustrated on p. 297) differs markedly from a sea-urchin,star-fishor brittle-star, in that the mouth faces upwards; the vent being also mi the uppersurface. This position is due to the fact that, so far as we know, all crinoids are 294 ECHINODERMS. at some time of their lives attached by a stalk to the sea-floor, or some other object,so that the mouth and vent naturally move up to that side of the body furthestfrom the stalk. This fixed state of existence has also caused the development ofarms, five in number, but often forked many times, which arms stretch out fromthe body on all sides of the mouth, and contain extensions of the nervous, blood-vascular, water-vascular, and generative systems. The representatives of the tube-feet are arranged along the sides of these arms, on their upper or oral surface, andbetween them is a groove, which is lined at the bottom with cilia, or extremelyminute hair-like processes, that keep waving in the direction of the mouth, and so
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GROUP OF STONE-LILIES (PeNTACKINIDS). maintain a constant stream of water towards the latter; such water containing theminute animalculse and fragments of decaying organic matter on which the crinoidfeeds. The extinct cystids and blastoids have their mouth in a similar position tothat of the crinoids, and for a similar reason, but have not similarly branchedarms. In the blastoids five grooves radiate down the body from the centralmouth, and from the sides of these grooves there spring small, jointed, butunbranched processes, called pinnules. The stem of the blastoids is very short,so that when the pinnules have been lost, as is usually the case, the five-groovedbody looks like a bud, whence the name of the class. It is difficult to describe acystid as having any definite shape, for the various animals to which this name isapplied differ greatly from one another in structure. Echinoderms are built upon CYSTIDS. 295 one or other of the plans of structure just described. Moreover, the an

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