Journal of conchology . he shell-gland, an organ of extreme importancein the Pelecypoda, appears, and the embryo—or more correctlyspeaking the larva—shews as a broadly pear-shaped body, almostindistinguishable from the marine larvae of Modiolaria or is the Trochophore stage, which in Sphceriuvi, Pisidmm, andGastropods is passed through in a more or less modified form withinthe egg-membrane, but is preserved as a free-swimming or veliger stagein the case of Dreissensia. Until their discovery by Dr. Korscheltveliger larvae were quite unknown in fresh water. The development of the la
Journal of conchology . he shell-gland, an organ of extreme importancein the Pelecypoda, appears, and the embryo—or more correctlyspeaking the larva—shews as a broadly pear-shaped body, almostindistinguishable from the marine larvae of Modiolaria or is the Trochophore stage, which in Sphceriuvi, Pisidmm, andGastropods is passed through in a more or less modified form withinthe egg-membrane, but is preserved as a free-swimming or veliger stagein the case of Dreissensia. Until their discovery by Dr. Korscheltveliger larvae were quite unknown in fresh water. The development of the larva goes on until the shell-membrane hasconsiderably increased in size, and presents the appearance of thebivalve mussel shell (fig. i, s.). This condition continues for a longperiod, during which the characteristic velum, a fleshy organ fringed withcilia at its margin, appears. By means of the expanded velum, andciliary movement, the larva can move very rapidly; and often when STANDEN : THE ZEBRA-MUSSEL. 87 Fig. I:. Fig. 1 —Young lar\a oiDi-eissensin, with bivalve shell {s.) and velum (rf/.), seen from the side,a, anus ; /«, stomach ; inu, mouth ; st?i, abductor muscle (from Korschelt, 92). swimming at full speed may be seen to stop suddenly and impart a spinning motion to its velum. At the slightest disturbance the velum retracts; the valves of the shell, which are only slightly open, snap Fig. 2. Fig. 3. s.
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Keywords: ., bookauthorconcholo, bookcentury1800, bookdecade1870, bookyear1879
