630 



MOLLUSC A CLASS IV. GASTROPODA. 



The spiral 

 the suture. 



---APEX 



COLUMELLA 

 UMBILICUS 



-ANTIOa CANAl 



Fig. 12. TOPOGRAPHY OF A GASTRO- 

 POD 



channel formed by the junction of the whorls is termed 

 A spiral line traced along the whorls midway between 

 the sutures would mark the periphery. 

 The mouth or aperture has sometimes 

 quite a circular margin (peristome) when it 

 is said to be continuous ; more often it is 

 interrupted, the side next the columella 

 being bounded by the inner wall of the 

 body- whorl only, when the exterior portion 

 is known as the outir lip or labrum, and the 

 other side as the -inner, columella lip or 

 labium. There is frequently a notch in the 

 margin of the aperture near the junction 

 of the outer lip with the body-whorl, and 

 another where it joins the anterior end 

 of the columella: these are respectively 

 termed the posterior and anterior canal, 

 and give passage to the siphons of the 

 animal (Fig. 12). 



The labrum is thin and sharp in most 

 immature shells, and in some adult forms; 



but more frequently it is either thickened, or curved outwards (reflected), 

 or curled inwards (inflected), or expanded, or fringed with spines. When 

 these thickenings or expansions occur periodically during the growth of the 

 shell they form conspicuous transverse markings on the whorl termed varices. 

 The exterior of the shell is also more frequently than not ornamented 

 with either spiral lines or ridges running in the direction of growth, or with 

 transverse markings coinciding with the "lines of growth," or with both. 



Many Gastropods, like the periwinkle, close the aperture of the shell when 

 they retire with a horny, or shelly trap-door, the operculum. It is developed 

 in the embryo within the egg, and to its inner surface are attached strong 

 muscles that hold it firmly in place. The following figures (Fig. 13) give some 

 idea of the principal forms. 



The head of the animal 

 bears either a single pair or 

 two pairs of tentacles, be- 

 neath which, quite in front, 

 the mouth is situated. Of 

 the armature of the mouth 

 mention has already been 

 made (ante, p. 612-14). 



The visceral mass is ex- 

 ternally covered by the 

 mantle, which is in one 

 piece. Towards the front, 

 and a little to the right side, 

 there is, under the edge of 

 the mantle, a cavity in 

 which are placed the gills 

 or gill, whilst into it also 

 the excretory duct and the 

 The mantle margin is sometimes 



Fig 13 PRINCIPAL FOBMS OF OPKRCULA. 

 A, Unguiculate or claw-shaped. B, Concentric. 

 C, MultispiraJ. D, Pancispiral. E, Articulate. 



terminal end of the digestive tract open. 



