. Outlines of zoology. s Dart Sac, and contains a pointed calcareousarrow (spiculum amoris), which is jerked out previous tocopulation. The dart is sometimes found adhering to thefoot of a snail, and after copulation the sack is empty, soon,however, to be refilled. When two snails pair, the genital apertures are dilated, the protruded penis of one. V is inserted into the ?^^ aperture of the other,and the spermatophoreof each snail is trans-ferred to the recepta-culum of the other. The large eggs are laidin the earth in Juneand July. Each is sur-rounded by gelatinousmaterial acquired in theovid
. Outlines of zoology. s Dart Sac, and contains a pointed calcareousarrow (spiculum amoris), which is jerked out previous tocopulation. The dart is sometimes found adhering to thefoot of a snail, and after copulation the sack is empty, soon,however, to be refilled. When two snails pair, the genital apertures are dilated, the protruded penis of one. V is inserted into the ?^^ aperture of the other,and the spermatophoreof each snail is trans-ferred to the recepta-culum of the other. The large eggs are laidin the earth in Juneand July. Each is sur-rounded by gelatinousmaterial acquired in theoviduct and by an elasticbut calcareous shell. Segmentation is totalbut slightly unequal. Asthe snail is a terrestrialGasleropod, there is notrochosphere larva, nor more than a slight hint of the char-acteristic Molluscan velum. A miniature adult is hatchedin about three weeks. The study of development may bemore profitably followed in the pond-snail Limnceus, wheregastrula, trochosphere, and veliger can be readily Fig. 212.—Diagram of larva of Palu-dina.—After Erlanger. Ec, Ectoderm; En., endodermj v., velum,with cilia; g.^ gut-cavity; , segmenta-tion cavity; , coelom pocket from gut;, blastopore groove closed, except atbl., which becomes the anus. The originof the mesoderm from a gut-pocket has asyet only been described in Paludina amongMollusca. Second Type of Mollusca. The Fresh-water Mussel{Anodonta cygnea), one of the Lamellibranchiata Habit.—The fresh-water mussel lives in rivers and lies with its head end buried in the mud, or movesslowly along by means of its ploughshare-like foot. Its food FRESH-WATER MUSSEL. 393 consists of minute plants and animals, which are wafted inat the posterior end by the currents produced by the cili-ated gillg. What is noted here in regard to Anodonta willalso apply, for the most part, to Unio and other fresh-watermussels. External appearance.—The bivalve is 4 to 6 in. long;its valves are equal and unite
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
