A manual of the Mollusca, or, A rudimentary treatise of recent and fossil shells . cyclas), or in the in-terspaces of the external branchial laminae (as inunio). At first they have a swimming disk, fringedwith long cilia, and armed with a slender ten-tacular filament ijlagellum). At a later period thisdisk disappears progressively, as the labial palpiare developed; and they acquii*e a foot, and with^ it the power of spinning a byssus. They now tially dissolve, wHlst the egg remains in the ovary, and before impregnation;it then passes to the centre of the yolk, and undergoes the changes describ
A manual of the Mollusca, or, A rudimentary treatise of recent and fossil shells . cyclas), or in the in-terspaces of the external branchial laminae (as inunio). At first they have a swimming disk, fringedwith long cilia, and armed with a slender ten-tacular filament ijlagellum). At a later period thisdisk disappears progressively, as the labial palpiare developed; and they acquii*e a foot, and with^ it the power of spinning a byssus. They now tially dissolve, wHlst the egg remains in the ovary, and before impregnation;it then passes to the centre of the yolk, and undergoes the changes describedby Barry, along with the yolk, whilst the nucleus of the germinal vesicle, orsome body exactly resembling it, is seen occupying a small prominence onthe sm-face of the vitelline membrane, until the metamorphosis of the yolkis completed, when it disappears, in some unobserved manner, without ful-filling any recognized purpose. * Fig. 30. Very young fry of crenella marmorata, Forbes, highly magni-fied ; d, disk, bordered with cilia:/, flagellum; v v, valves; m, ciliated STRUCTURE AND PHYSIOLOGY OP THE MOLLUSCA. 53 have a pair of eyes, situated near the labial tentacles (fig. 30*, e\which are lost at a further stage, or replaced by numerous ru-dimentary organs placed more favourably for vision, on the bor-der of the mantle. Most of the aquatic gasteropoda are very minute whenhatched, and they enter life under the same form,—that which
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Keywords: ., bookcentury1800, bookdecade1850, booksubjectmollusks, bookyear185
