External Morphology
The external morphology of Scintilla spp. is particularly distinctive given the unusual structure of the mantle. The mantle of bivalves is generally located within the interior of the shell, covering the visceral mass. The mantle skirt encloses the body and the mantle cavity, ending at the mantle margin, which is typically at the edge of the shell. In Scintilla spp. however the mantle skirt extends to the exterior and reflects entirely over the outside of the shell as shown in Figure 1 and 2.
   
Figure 1: The exterior of Scintilla spp.
Figure 2: Diagram illustrating the position of the visceral mass and the external mantle of Scintilla spp. Adapted from Ruppert Fox and Barnes
The mantle plays an important role in the growth of molluscans as it is involved in the secretion of the shell. The mantle margin has three folds, the outermost of which has the mantle epithelium which is responsible for the excretion of the shell layers. A Molluscan shell is composed of three layers; the periostracum, the ostracism and the hypostracum. The protein is secreted initially in soluble form then hardens into a dark, insoluble layer. Growth of the shell continues throughout life through the secretion of Calcium, salts and proteins into a space between the mantle epithelium and the periostracum before being deposited into the shell.The oldest part of the shell is the umbo and growth rings can be seen radiating out from it (Figure 2).
 
Figure 2: Internal shell of Scintilla spp. showing concentric growth rings
Some degree of mantle fusion is evident on Scintilla spp. as is shown in figure 3. Mantle fusion typically occurs in one or all of three regions where openings are not necessary. This tactic is primarily used by burrowing bivalves in an attempt to reduce the volume of sediment entering the manta cavity while burrowing however it is seen in non-burrowing species, such is the case here to reduce general waste from entering the mantle cavity.
In order to examine the anatomy of Scintilla spp. and determine the function of the extension of the mantle, specimens were preserved in fixative and brought back to St Lucia. A sample of the exterior mantle was sectioned vertically providing a series of cross-sectional views (see Figure 4).
 
Figure 4: Cross-sections of the exterior mantle of Scintilla spp. at magnification
The images displayed in figure 4 clearly demonstrate a smooth and a convoluted side to the portion of the mantle that reflects over the exterior of the shell. This is consistent with current knowledge on mantle design in which the convoluted side secretes the compounds required to build and grow the shell while the smooth side is composed of striated muscle. The presence of the convoluted side on the exterior mantle indicates that in Scintilla spp. the function of the extension of the mantle to reflect over the shell exterior may be to grow the shell from both sides. This would increase the strength of the fragile shell which is a characteristic of Scintilla (Mather 1993).
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