Snail Structure
The features of the structure of the snail shell are important to identification. This section is an overview of the major features of the snail shell. But first, the whole snail body. When you see a live snail, you are missing most of the body - it is inside. You can see the muscular gliding foot and the head with its two pairs of tentacles. The upper pair contain the eyes and the lower pair are general sensory tentacles. The rest of the animal’s organs are inside the shell. This snail is a Polygyrid and you can see the reflected lip with the mantle at the edge. The mantle secretes the shell.
For some few species, the color of the body is important for identification. Most bodies tend to tan, light brown, to dark.
This snail head shot shows the two pairs of tentacles head-on. The upper tentacles house the eyes. The mouth is in between the lower sensory tentacles and just below. It contains a muscular strip of tissue with rasping teeth – called a radula. The snail cruises along scraping the surface and eating what is there.
The body of a slug is out there for all to see and is important for identification to species. On this Giant Garden Slug, you can see the slightly lighter-in-shade spotted mantle at the front (anterior) end and the longitudinally striped body behind. That mantle and the location of the pneumostome (breathing air hole) are among the useful characters for identification. These are covered in the key for slugs.
The Land Snail Shell
The major features used include the presence/absence of a visible shell, shape of the shell, the type of umbilicus, the nature of the edge of the outer lip of the aperture, the presence/absence of various teeth in the aperture, surface features, and whorl patterns and counts.
This is the general external anatomy of the shell as seen from the apertural side of a heliciform snail, the Flamed Tigersnail.
As the snail grows and new shell is added, it can grow more quickly “up” than across – giving a taller than wide shell – or less quickly – giving a wider than tall shell. Note that the images below are not to scale!
1. Shell Morphology - Shell Shape
Taller than Wide Shells
Succiniforms have a very large aperture and unique spiral (Family Succiniedae). Pupilliforms have more linear sutures (lines that proscribe the whorls) in apertural view (as pictured) and tend to be slightly “chunky” (Gastrocopta, Vertigo, Columella) although a few are slimmer and taller by comparison. The fusiform shell, on the other hand, is more streamlined (Cochlicopa and Carichium). There are no bulimoid-shaped snails in the databases for Indiana, but the Whitewashed Rabdotus has been found just to the west in Illinois.
Wider than Tall Shells
Three types of these can be seen: typical heliciform, pill-shaped, or dome/beehive shaped.
Heliciform – this is the most common group of wider than tall snails and the shell profile varies from globose (more circular) to subglobose (more oval, sometimes called “slightly depressed”) to depressed (flatter but still a small peak) and ultimately to discoidal (flat like a cinnamon roll). These all represent variation in how high the apex grows and how round the whorls appear. Since the appearance, especially of globose, subglobose, and depressed is a gradient and may depend on personal judgement, a range is usually given in the keys.
Pill-shaped – There are two genera of pill-shaped snails and one almost-pill-shaped. The shell is squat, the aperture is vertically narrow, but wide horizontally with a long parietal tooth.
Shell beehive (Euconulus) or domed shaped (Strobilops, Ventridens) – since these represent two groups of small snails, they will be easy to pick out when you see them.
2. Shell Morphology - Form of the Umbilicus
The umbilicus represents the center of the spiral form as seen from the base. It may be closed (covered over, actually) or open so wide that you can see up clear to the bottom of the first whorl. The forms include imperforate (no hole, or rather it is completely covered by a callus associated with the lip); perforate (a very small hole); rimate (partially covered by a callus so that you can still see the umbilicus); umbilicate (a “good sized” umbilicus) to widely umbilicate.
3. Shell Morphology - Edge of the Aperture (the lip or peristome)
This is the outer edge of the last whorl. Un-reflected lips (a Tigersnail here) remain unmodified with maturity, just a clear edge. A reflected lip (a Mesodon here) is rolled back and thickened upon reaching maturity. Some unreflected lips are simple and smooth but some may be thickened a bit – just not reflected. Mature species are easy to distinguish based on this.
The problem is that those species with reflected lips when mature, have un-reflected lips as juveniles. So, does that shell with an unreflected lip there in your hand belong to a species with an unreflected lip or is it an immature of a larger species whose lip will reflect when larger? Usually, but not always, you will find immature specimens in a collection of reflected adults. Chances are it is the same species. Solo specimens are a challenge at first, but with some experience, with more species, you’ll be able to distinguish these problem specimens based on the overall shape of the shell, the number and the size of whorls, and its umbilicus (but not always).
4. Shell Morphology - Apertural teeth (lamellae or denticles)
As the snail grows it can put down thickened layers of shell that can look like “teeth”. They are also called lamellae or denticles. The apertural teeth are a variety of projections in terms of number, size, and shape. The parietal tooth is found on the body wall of the aperture facing the outer lip. It may be absent or present in various forms such as short and stub-like or longer and curved. Palatal teeth are found on the upper and outer lip margins. On the lower rim of the lip there may be a basal tooth (as a narrow peg or a longer bar or ridge).
Some Vertiginidae, tiny snails, have more teeth than large snails. There are a pair of fused teeth nearest the body whorl - the angular and parietal. These may be separate, fused as in the image, or absent (one or both). The tooth on the left is the columellar tooth, located inside the aperture, growing from the columellar. The remaining teeth are the basal and palatal teeth.
5. Shell Morphology - Shell Surface
As the shell grows, surface features may be added to it. Surface features may include radial ribs (A and B, perpendicular to growth) or fine lines called striations that are parallel to growth (C). Growth may leave faint radial growth lines or “wrinkles” (D). In some cases, there may be radial lines or growth wrinkles (D). These may be intersected by spiral (seen with eye or microscopic) indented or raised lines (E and F). Some snails have tiny bumps called papillae (G). The shell may also be glossy and smooth or dull or may have hairs. Some magnification is needed to see many of these features. A 10x loop can help see the most prominent.
6. Shell Morphology - Whorl Patterns and Counts
Whorls are the successive turns of the shell during growth.. Not all additions are the same. In some cases (A, C), the growth is relatively regular while in others (B, D) the later whorls the increase in size greater. A is a Mesodon and B is a Mesomphix. Mesomphix never reflects a lip, but Mesodon does when mature. As a result both look superficially alike when juveniles - but if you look at the whorl growth you should be able to tell the difference. C is Zonitoides with a whorl pattern like Mesodon, and D is Glyphylinia with a whorl pattern more like Mesomphix. Note that Glyphylinia also has nice clear radial lines. The vertical lines at the base of each image are mm markers.
The next image is a growth series of Neohelix. You can see the that whorls are added as the shell has grown. the third Neohelix from the left is about the same size as Zonitoides above (C) and has fewer whorls. That's something to keep in mind when finding smaller snails (6 mm or less).
Counting whorls is often needed, as is counting the number of ridges or ribs in the last whorl (also called the body whorl). Here is how.
For the whorl count, look closely at the apex of the shell and identify the semi-circle that defines the start of the first whorl (inset of following image). Draw a line (imaginary if the shell is on your hand) perpendicularly across base of the semicircle that defines the start of growth. Extend that line outwards. Now, follow the whorl and each time you've completed a 360-degree turn, you've added a new whorl (larger image). There is probably about a 1/4 or more error built into the partial whorl (the last, or body whorl) depending on how well you make that line.
For ridge counts and the like, just count from the point where at the end of new growth against the whorl (A) all the way around the outside edge to the end (B).
GLOSSARY
Aperture – the opening of the shell, which varies from circular to ovate.
Apex – the tip of the shell
Body whorl – the last whorl, ending in the aperture
Columella – think of this as the central column around which the spiral whorls form; like the center pole of a spiral staircase
Mantle – outer tissue layer of the body; it secretes and makes the shell and shell features like the teeth
Nuclear whorls (protoconch) – the first spirals of the shell, formed in the egg
Outer lip – the end of the last whorl forms the aperture, the outer edge is the outer lip, also called the peristome; it may be reflected (bent backward) or unreflected (simple), thickness can vary
Parietal wall – the inner part of the last whorl, seen from the aperture
Spire – refers to the elevation of the shell as the whorls grow downward; some spires are high, others low
Suture – the margin between adjacent whorls; ranges from being V-shaped to smooth
Umbilicus – the center of the whorls, as seen from below, may be large, small or non-existent
Whorl – the turning of the spiral shell, lengthens and usually widens as the animal gets larger