GASTEROPODA. 101 



fn acidulous waters (e. g. cerithidea wcA.pirena}, and specimens may be ob- 

 tained from more favorable situations with the points perfect. 



The line or channel formed by the junction of the whirls is termed the 

 suture. 



The last turn of the shell, or body -whirl, is usually very capacious ; in 

 the females of some species the whirls enlarge more rapidly than in the males 

 (e. g. buccinum undatum). The "base" of the shell is the opposite end to 

 the apex, and is usually the front of the aperture. 



The aperture is entire in most of the vegetable feeders (holostomata], but 

 notched or produced into a canal, in the carnivorous families (siphonostomata}; 

 this canal, or siphon, is respiratory in its office, and does not necessarily indicate 

 the nature of the food. Sometimes there is a posterior channel or canal, 

 which is excurrent, or anal, in its function (e. g. strombida and ovulum volva] ; 

 it is represented by the slit in scissurella, the tube of typhis, the perforation 

 infissurella, and the series of holes in haliotis. 



The margin of the aperture is termed the peristome ; sometimes it is con- 

 tinuous (cyclostoma), or becomes continuous in the adult (carocolla) ; very 

 frequently it is " interrupted," the left side of the aperture being formed only 

 by the body-whirl. The right side of the aperture is formed by the outer 

 lip (labrum), the left side by the inner or columellar lip (labium), or partly 

 by the body- whirl (termed the "wall of the aperture" by Pfeiffer). 



The outer lip is usually thin and sharp in immature shells, and in some 

 adults (e. g. helicella and bulimulus] ; but more frequently it is thickened ; 

 or reflected ; or curled inwards (inflected*), as in cypraa; or expanded as in 

 pteroceras ; or fringed with spines as in murex. When these fringes or ex- 

 pansions of the outer lip are formed periodically they are termed varices. 



Lines of colour, or sculpture, running from the apex to the aperture are 

 spiral or longitudinal, and others which coincide with the lines of growth are 

 " transverse," as regards the whirls ; but stripes of colour extending from 

 the apex across the whirls are often described as "longitudinal" or "radia- 

 ting," with respect to the entire shell. 



Shells which are always concealed by the mantle are colourless, like Umax 

 and parmophorus ; and those which are covered by the mantle-lobes when 

 the animal expands, acquire a glazed or enamelled surface, like the cowries ; 

 when the shell is deeply immersed in the foot of the animal it becomes partly 

 glazed, as in cymba. In all other shells there is an epidermis, although it is 

 sometimes very thin and transparent. 



In the interior of the shell the muscular impression is horse-shoe shaped, 

 or divided into two scars ; the horns of the crescent are turned towards the 

 head of the animal. 



The operculum with which many of the gasteropods close the aperture of 

 their shell, presents modifications of structure which are so characteristic of 

 the sub-genera, as to be worthy of particular notice. It consists of a horny 

 layer, sometimes strengthened by the addition of calcarious matter on its ex- 



