MOLLUSCA PltOrEK. 



203 



organs may be in tlie form of plume-like gills, contained in a more or 

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

 In many members of this grtmp the water 

 attains access to the gill-chamljer by means 

 of a tubular prolongation or folding of the 

 mantle, foi-niing a sii>hon (lig. 140, s), and 

 often the effete water is expelled by another 

 tube which is similarly cunstnicted. In the 

 second great section — often called the " ))ul- 

 monate" Gasteropoda — respii'ati(jn 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 (Jasteropoda are mostly 

 distinct, but in some groups ('-'.,'/■, 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, wdiich 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 



Fig. Ul.— PortitMi of the 

 lingual ribbon of the 

 I' iiiiinon Whelk, Tiiagrii- 

 licd (after "Woodward). 



Fig 142.— Gastfiroiioda. Tlie Garden Snail {Helix nsptrsa). 



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

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

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

 As reo-ard.s' the shell of the Oiisteropodn, the following points may 



