Page 14 - Land Snails of New Mexico

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oftentacles.When theanimalisfullyextended,acollaris observed around the edge of the aperture. This is the edge of the mantle, which, posterior to the collar, is hidden from view. In N e w Mexico slugs, however, which lack an external shell, the mantle is seen as a sort of beret perched atop the animal in the anterior one-third of its body. The mantle secretes the shell, which consists of several layers. T h e outermost is mainly of the organic, noncalcareous compound conchiolin. It is often pigmented--commonly some kind of brownish color in N e w Mexico land snails. The periostracum may be rubbed off (exfoliated) in older shells of living animals or in shells remaining after the living occupant has died.
whorl,witheachwhorlgirdlingashell360°.Theoldestwhorl is at the apex, and the youngest terminates at the aperture. M o s t N e w Mexico land snails hatch from eggs, but a few are born as baby snails. In either case, the minute shell that they already have produced prior to hatching, or birth, comprises the embryonic shell (also referred to as nuclear whorls or the protoconch) of about 1 % - 2 % minuscule whorls. These whorls comprisetheuppermost,closedendoftheapexofthecone.The
appearance oftheembryonic whorls may be noticeably different fromthatofthelaterwhorlsincolorandornamentationandmay be ofuseinidentifyingsome kindsofsnails.
(Terms stressed in this and the next paragraph are illustrated in Fig. 2A). T h e youngest whorl terminates at the aperture, and iscalledthebody whorl. Itisoftenmore capaciousthanearlier whorls. It extends around the shell from the outer edge of the aperture back and up 360° to the location on the preceding whorl that isjust above the outer lip--where we started. The stack of whorls above the body whorl is referred to as comprising the spire of the shell by most authors, and is so-used herein. However, Burch (1962:13), in his widely used handbook of eastern land snails, illustrated the spire as extending from the upperendoftheaperturetotheapex.The spirecanberelatively highorgradetononexistentintheflattestshells.The continuous
spiral groove that marks the junction of one whorl with its neighbor, above or below, is called the suture. In practical terms, it is the indented groove that w e note between whorls
, whenlookingatashellinaperturalorintopview.Suturesmay be deeply indented (impressed) or more shallow.
Ifwe lookatashell,facingtheaperture,theverticaldistance f r o m t h e b o t t o m o f t h e a p e r t u r e t o t h e a p e x is c o m m o n l y c a l l e d theheightoftheshell,thetermwe usehere,althoughsome prefer to use the term length for this dimension. Likewise, still facing the aperture, the greatest distance from one side to the other of the shell is the width (the term w e use) or diameter of the shell. In land snails, the greatest width will be the dimension from the farthest extension outward of the aperture, across the shell 180° to its opposite side. In flat shells, width may be observed most easily from a top view. In s o m e snails, height m a y be greater than width and in others, width greater than height. Thisdistinctionisemployedinthetaxonomic key (seebelow) to divide New Mexico land snails into two major groups for purposes ofidentification.
To return to our model of the shell as a spiraling, hollow cone,we notethatinhigher,narrowershellsthewhorlsspiral, in succession, around a central axis formed of the inner sides of each whorl. This central axis is called the columella (Fig. 2A). The columella (littlecolumn) originates as a hollow structure, b u t i n s o m e k i n d s o f s h e l l s it m a y fill w i t h c a l c a r e o u s m a t e r i a l later on to form a true column or pillar. In such a shell, a whorl thatisslicedapartatany levelwould show thatitsinnerwall is contributing to the columella. T h e aperture provides, in effect, such a sliceacross a whorl, and itscolumellar wall will be on the sideoppositetheouter,freeliporwalloftheaperture(Fig.2E). .
Attached to the columella is the columellar muscle, used in retracting the animal into its shell. The columella may bear various kinds of plaits or lamellae, which aid in attachment of the columellar muscle. The cavity encompassed by the columellar walls of the shell is called an umbilicus. In high,
Figure1.A,Cone,whichmayneverhaveexistedasashellformin phylogeny of the gastropods, but can serve as a model for understanding development of the gastropod shell, as suggested by Ellis (1926), B,Likewise,shellsofthemarinewormshells(family Vermetidae) provide a model for a shell with loosely coiled cone. C, Tightlycoiledshell(familyPupillidae)typicalofmany ofthelandsnails ofNew Mexico.
The Shell
A modelsuggestedbyEllis(1926:15)inhisbookBritish
Snailselucidatesthestructureofsnailshells.Elliscompared the
shelltoahollow,elongatecone(Fig. 1A)thatbecomesspirally coiled"formechanicalconvenience."Theterm"helicocone"has
been applied to this structure. The uppermost (closed) end of such ahelicocone would correspond to the apex of a snail shell, while the lowermost, open end would correspond to the aperture,amouth-likeopeningoftheshell.From theapex,the conical tube down-spirals (Fig. 1C). The model is well illustratedbysomemarineshellscalledWorm Shells(genus Vermicularia), in which lower spirals are coiled loosely and not attached to each other (Fig. IB). In our land snails, however, down-spiralling takes place around a central axis in such a w a y t h a t e a c h l o w e r c o i l o v e r l a p s t h e o n e a b o v e it t o s o m e e x t e n t . I n disc-shaped shells (discoid), each lower coil is progressively muchwiderthantheoneaboveit,thusallowingmuchofthe undersides of the older whorls to be exposed in a wide umbilicus, further discussed below. Each coil or spiral is a