MOLLUSCA PROPER. 



203 



organs may be in the form of plnme-like gills, contained in a more or 

 less complete chamber, formed by a folding of the mantle (fig. 129). 

 In many members of this group the water 

 attains access to the gill -chamber by means 

 of a tubular prolongation or folding of the 

 mantle, forming a siphon (fig. 140, «), and 

 often the effete water is expelled by another 

 tube which is similarly constructed. In the 

 second great section — often called the " pul- 

 monate" Gasteropods — respiration is effected 

 by a pulmonary chamber or lung, formed by 

 a folding of the mantle, and having air ad- 

 mitted to it by a distinct aperture. 



The sexes in the Oasteropoda are mostly 

 distinct, but in some groups {e.g., the Pul- 

 monate forms and the Sea-slugs) they are 

 united in the same individual. The young, 

 when first hatched, are always provided with 

 an embryonic shell, which may be entirely lost in the adult, or may 

 simply become concealed by a fold of the mantle. In the water- 

 breathing forms the young is protected by a small nautilus-shaped 



ig. 141.— Portion of the 

 lingual ribbon of tbe 

 common Whelk, magni- 

 fled (after Woodward). 



Fig 142.— Gasteropoda. The Garden Snail (Helix aspersa). 



shell, within which it can entirely withdraw. At this stage, it is 

 enabled to swim about freely by means of two ciliated lobes spring- 

 ing from the sides of the head, but this ciliated '' veil " is soon lost. 

 As regards the shell of the Gasteropoda, the following points may 



