. The sea-beach at ebb-tide : a guide to the study of the seaweeds and the lower animal life found between tidemarks . the free maiit le-ed^es haveu way of adjusting themselves posterioi-ly to form fum-liuinilsi/ilimis without actually Coalescing. 1sually, however, the mantie Haps not only unite posteriorly to form true siphons, but arecapable at that point of varying decrees of protrusion from theshell, and when extended the siphons appeal- as two tulies. Insome genera these siphonal tulies are very lon^; in otlnTr- theyare fastened together and surrounded liy a t«>u--h. h-athei-y intejrn-
. The sea-beach at ebb-tide : a guide to the study of the seaweeds and the lower animal life found between tidemarks . the free maiit le-ed^es haveu way of adjusting themselves posterioi-ly to form fum-liuinilsi/ilimis without actually Coalescing. 1sually, however, the mantie Haps not only unite posteriorly to form true siphons, but arecapable at that point of varying decrees of protrusion from theshell, and when extended the siphons appeal- as two tulies. Insome genera these siphonal tulies are very lon^; in otlnTr- theyare fastened together and surrounded liy a t«>u--h. h-athei-y intejrn- 412 MARINE INVERTEBRATES ment, which, like the siphons proper, is only an extended portionof the mantle-edge. The orifices of the siphons are generallypapillaceous. These two siphons (for there are always two ifthere are any at all) are the anal or excurrent (upper) and thebranchial or incurrent siphon. The function of the latter is todraw in the pure water to bathe the gills and to furnish food,while the office of the former is to eject waste materials and thewater which has already passed over the gills. Diagram illustrating the various degrees of union of the mantle lobes: A, mantle completelyopen; B, rudiments of siphons, mantle still completely open; C, mantle closed at one point; D,mantle closed at two points, with complete formation of siphonal apertures; E, development ofsiphons, ventral closure more extended; P, mantle closed at three points, with fourth oriftce; /,foot; , s. b., anal and branchial siphons; 1, 2, 3, tirst, second, and third points of closure ofmantle; 6. o., byssal aperture. MANTLE FUSION Besides the points of juncture where the mantle-edges uniteto form the siphons, the coalescence of the two mantle lobes maybe extended, and they may become further united and fused toge-ther at other points. Indeed, the fusion of the mantle-edges maybecome almost complete; but it always leaves the siphonal open-ings and a third opening through which the foo
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectmarinea, bookyear1901
