. A manual of zoology. Zoology. II. ACEPHALA 319 excavated at the posterior end, so that when brought together two open- ings, an upper and a lower, resuk (fig. 320, C). The lower of these is the branchial opening by which fresh water passes into the mantle cham- ber; it flows out after passing over the gills, along with the fxces, through the upper or cloacal opening. In many bivalves the free edges of the mantle grow together, leaving three openings (fig. 320, B), one for the protrusion of the foot, the others the two just described, now called the branchial and cloacal siphons. By further d
. A manual of zoology. Zoology. II. ACEPHALA 319 excavated at the posterior end, so that when brought together two open- ings, an upper and a lower, resuk (fig. 320, C). The lower of these is the branchial opening by which fresh water passes into the mantle cham- ber; it flows out after passing over the gills, along with the fxces, through the upper or cloacal opening. In many bivalves the free edges of the mantle grow together, leaving three openings (fig. 320, B), one for the protrusion of the foot, the others the two just described, now called the branchial and cloacal siphons. By further development the margins of these openings are drawn out into two long conjoined siphonal tubes (A), which for their retraction need special muscles; these are attached to the valves and thus cause the pallial sinus referred to above. In the shell there are three layers (fig. 321): on the outside a thin organic cuticula and below two layers largely of calcic carbonate. In many these two layers are distinguished as the prismatic layer and the nacreous laver, the first consisting of closely packed prisms; the nacreous layer of thin lameilie generally. Fig. 321.—Section of shell of Anodonta. c, cuticula; p, prismatic layer; I, nacreous layer. parallel to the surface. These produce diffraction spectra and so the iridescent appearance of the shell; the finer the lines tlius formed the more beautiful the play of colors. This is especially noticeable in the mother-of-pearl shell Melca- grina and many Unionidas. When foreign substances get between mantle and shell they stimulate a greater secretion of nacreous substance and become surrounded by layers of it. Pearls are formed in this way. The gills lie between the mantle and the body and from their lamellar character have given rise to the name Lamellibranchiata (figs. 322, 323). Two gill-leaves occur on either side, although occasionally the outer or both may degenerate. Frequently the inner gills of the two sides unite behind the bod
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectzoology, bookyear1912
