. Elementary text-book of zoology. Zoology. GASTROPODA. 31 lobes or drawn out into processes. The lower surface of the mantle usually serves as the roof of a cavity, which extends on to the dorsal surface and also on to the sides of the body. This cavity contains the respiratory organ, and opens to the exterior by an aperture or tubular prolongation at the mantle edge. The body cavity is developed on the dorsal surface of the foot,, usually in a visceral sac, which projects like a hernia. The visceral sac tapers gradually at its upper end, and is usually spirally twisted. The mantle and viscer
. Elementary text-book of zoology. Zoology. GASTROPODA. 31 lobes or drawn out into processes. The lower surface of the mantle usually serves as the roof of a cavity, which extends on to the dorsal surface and also on to the sides of the body. This cavity contains the respiratory organ, and opens to the exterior by an aperture or tubular prolongation at the mantle edge. The body cavity is developed on the dorsal surface of the foot,, usually in a visceral sac, which projects like a hernia. The visceral sac tapers gradually at its upper end, and is usually spirally twisted. The mantle and visceral sac are covered by the shell, which to a certain extent repeats the twistings of the latter and can usually completely receive and protect the head and foot when the animal is retracted. The shell is as a rule hard and calcareous, and possesses an internal nacreous layer similar to that of the mother-of-pearl layer of the Lamellibraiich shell. The shell is sometimes delicate, horny, and flexible, or it may have a gelatinous (Tiedmannia) or cartiliginous (Cyinlndiii) consistency. More rarely the shell is so small that it only covers the mantle cavity with the respiratory organs or lies hidden completely within the mantle (Limax, Pleurobranchiata). In other cax-s it is thrown off at an early stage, so that the adult beast is completely without a shell (Nudibranchiata}. The shell differs from that of the LctynelKbranchiata in being composed of a single piece; it is either flat and cup-shaped (Patdhi) and uncoiled, or it is spirally twisted in very different ways, from a flat disc-shaped to the long drawn-out turret-shaped spiral (fig. 509). In the first case it more resembles the embryonic shell, which lies as a delicate, cap-shaped covering 011 the mantle. The growth of the shell keeps pace with that of the animal, the additions being made to the edge of the shell, viz., to that part which lies on the edge of the mantle. In consequence of the inequality of this growth spiral .
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
