Animal and vegetable physiology, considered with reference to natural theology, by Peter Mark Roget .. . t may even perform a slow lateralmovement. They descend in the water bysimply contracting their dimensions in everydirection. Sometimes, in order to sink morequickly, they turn themselves over, so that theirconvex part is undermost. Medusae are met with of very various sizes;the larger abound in the seas around our coast;but immense numbers of the more minute andoften microscopic species occur in every part of VOL. I. o 194 THE MECHANICAL FUNCTIONS. i the ocean.* In some parts of the Greenl
Animal and vegetable physiology, considered with reference to natural theology, by Peter Mark Roget .. . t may even perform a slow lateralmovement. They descend in the water bysimply contracting their dimensions in everydirection. Sometimes, in order to sink morequickly, they turn themselves over, so that theirconvex part is undermost. Medusae are met with of very various sizes;the larger abound in the seas around our coast;but immense numbers of the more minute andoften microscopic species occur in every part of VOL. I. o 194 THE MECHANICAL FUNCTIONS. i the ocean.* In some parts of the Greenlandseas they swarm to such an extent that theygive a visible tinge to the colour of the waves forhundreds of miles. The total number of theseanimals dispersed over that space surpasses theutmost stretch of the imagination. In thesesituations a cubic foot of water, taken indiscri-minately, was found by Mr. Scoresby to containabove 100,000 of these diminutive medusae. Belonging to the tribe of Medusaria is a sin-gular genus, denominated the Beroe, (Fig. 82and 83,) which is remarkable for its organs of. progressive motion. Its body is either globular,or oblong, and it swims with its axis in a verticalposition. Eight longitudinal bands or ridges,which have been sometimes compared to ribs, * The luminous property of sea water, or its phosphorescence,as it is sometimes called, generally arises from the presence ofminute medusae, which are met with in greatest numbers at thesurface, being specifically lighter than the surrounding fluid. BEROE. 195 extend down its sides, like those of a melon ;and along each of these is attached a set of littlemembranes, extended horizontally, and snp-ported on radiating fibres; so that they bear apretty exact resemblance to the fin of a action is not nnlike that of the wings of abird; for they are made to flap up and down,striking the water vertically, and communicatingan ascending impulse to the body. This animalis also provided w ith two
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