Animal and vegetable physiology, considered with reference to natural theology, by Peter Mark Roget .. . st insect that craw Is unheeded at our feet. 192 THE MECHANICAL FUNCTIONS. § 5. Acalepha. Floating masses of living gelatinoHS matterare met with in every part of the ocean; oftenin vast numbers, and of various forms ; and hav-ing but little the appearance of belonging to theanimal kingdom. They compose the order Aca-lepha, of which theMedusa (Fig. 81) maybe taken as the appear, fromtheir organization, tobe raised but a singlestep above polypi; andin point of activity andlocomotiv
Animal and vegetable physiology, considered with reference to natural theology, by Peter Mark Roget .. . st insect that craw Is unheeded at our feet. 192 THE MECHANICAL FUNCTIONS. § 5. Acalepha. Floating masses of living gelatinoHS matterare met with in every part of the ocean; oftenin vast numbers, and of various forms ; and hav-ing but little the appearance of belonging to theanimal kingdom. They compose the order Aca-lepha, of which theMedusa (Fig. 81) maybe taken as the appear, fromtheir organization, tobe raised but a singlestep above polypi; andin point of activity andlocomotive powers, theyrank among the lowestof those Zoophyteswhich are not permanently fixed to the spotwliere they were first developed. They arealmost wholly passive beings, floating on thesurface of the sea, or remaining at a small depthbelow it, carried to and fro by the motion ofevery tide and current, and destined to be therniresisting prey of innumerable tribes of animalswhich people every part of the ocean. The usual form of a Medusa is that of a hemis-phere, with a marginal membrane, like the fold. MEDUSA. 193 of a mantle, extending loosely downwards fromthe circumference; together with a central pe-dicle descending from the lower surface, like thestalk of a mushroom, and terminating below inseveral fringed laminae, or processes, which havesometimes been denominated tentacula. The whole substance of the body of thesemedusae is semi-transparent and gelatinous, with-out any distinct fibrous structure; yet it hasconsiderable elasticity, and possesses also somedegree of contractile power. The animal isseen alternately to raise and depress the mar-gin of its hemispherical body, and to flap withthe fringed membrane or mantle, which descendsfrom it, in a manner somewhat similar to theopening and shutting of a parasol. This pulsa-tory movement is performed about fifteen timesin every minute, with great regularity: and bythe reaction of the water, the animal is sus-tained at the surfa
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