. A manual of zoology. Zoology. o-'S MOLLUSCA iHis lavcr. contains flic piijmcnl and ma\- lie covered by a horny cnticlc. Tn rare cases mantle and shell are lacking;, or the mantle is present lint the shell is rtidi- mentarv and not xisible externally because the mantle folds ha\e ^jroun over it. In these cases the \isceral sac is not prominent. Since the shellless forms possess a mantle and shell in the young, the adult conditions are explained by degeneration. Only a few gastcropods arc bilaterally symmetrical. I'stially the spi-al twist of the visceral sac has resulted in a torsion of otJic
. A manual of zoology. Zoology. o-'S MOLLUSCA iHis lavcr. contains flic piijmcnl and ma\- lie covered by a horny cnticlc. Tn rare cases mantle and shell are lacking;, or the mantle is present lint the shell is rtidi- mentarv and not xisible externally because the mantle folds ha\e ^jroun over it. In these cases the \isceral sac is not prominent. Since the shellless forms possess a mantle and shell in the young, the adult conditions are explained by degeneration. Only a few gastcropods arc bilaterally symmetrical. I'stially the spi-al twist of the visceral sac has resulted in a torsion of otJicr parts from left to right, ill wliich alimcutary tract, tiephridia, gills, heart, ami iier\ous s_\-stem take part i^lig .^^j). Tiio intestine is bent in tliis wa\', the anus opening into the mantle chamber oil the right side (H) or the twisting mav be continued so far as to tlouble the ,. intestine on itself, anus being in the middle line ric. ^u.â^inis- ^ tral shelf of Lii!;i,\i in front, near the licad [O. Xepihridia, gills (willi Ciirmaius Uroni [hem the osphradia), and Iteart wamler in company, Lcunis). so that the organs pnmitiwdv belonging on the lelt side may be transferred to the right and ^â ice versa. With this there is a tendency to asymmetry and the loss of the organs (usuallv of the primitively left side). \Mien the nervous system takes part in the twist-. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Hertwig, Richard, 1850-1937; Kingsley, J. S. (John Sterling), 1854-1929, ed. and tr. New York, H. Holt
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectzoology, bookyear1912
