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 

 .twistings arise, the diameter of which gradually and continuously 

 increases. .Inasmuch as the unsymmetrical growth of the shell is 

 due to the unequal growth of the body, the position of the openings 

 of the unpaired organs (anus, sexual opening) to one side of the- 

 great external lip of the shell is intelligible. 



The following parts may be distinguished in a spirally-twisted 

 shell; (1) the apex, as the part of the shell at which the growth 

 began and from which the spiral twistings started; (2) the opening 



FIG. 509. Section through the 

 shell of II, /i.i- jifiMiitiu. 



