362 
MOLLUSCS. 
its prey whole, shell and all. The shell is then broken up by the aid of a very re¬ 
markable gizzard, and the soft parts digested. This gizzard consists of three strong 
calcareous plates of different size and form, which grind against one another by 
powerful muscles. The shells of Scaphander 
are mostly thin, spirally striated, oblong, 
convolute, without any visible spire, having 
the aperture narrow behind, and much wider 
in front. The known species are not 
numerous, and occur chiefly in seas of the 
Northern Hemisphere, but a few have been 
described from other parts of the world. 
larva of Entoconcha (much magnified). raundus was obtained at the Aiu Islands, 
at a depth of eight hundred fathoms, and 
S. gracilis off the Azores, in one thousand fathoms. By far the largest known 
form is the common British S. lignarius, which ranges from Finmark to the 
Mediterranean. 
The bubble-shells ( Bullidcc ), as this group is popularly called, are something 
like Scaphander in form, but rather rounder and stronger in their structure. 
Most of those belonging to the typical genus Bulla are prettily painted with 
blotches, clouds, and dots. The animal is not too large for its shell, has distinct 
eyes, and a different type of radula to that of Scaphander, which these molluscs 
resemble in their predatory habits. Another genus is represented in Britain by 
Acera bullata (twice nat. size). 
Acera bullata, which occurs on oozy ground and mud-flats in many estuaries. It 
also ranges from Finmark to the Mediterranean; and is remarkable for its thin,horny, 
convolute shell, with a slit at the suture, through which the animal protrudes a 
long, thread-like mantle-ajDpendage. The side-lobes are largely developed, and 
can be reflexed so as to completely envelop the shell. They are also employed in 
swimming. Another family is represented by Ringicula, all the members of 
which are very small, not a quarter of an inch in length. The shells are entirely 
white, more or less globular, with a pointed spire, and with the aperture—which is 
notched in front—to some extent contracted with folds. The channelled character 
of the aperture is rather exceptional, the shells of the Tectibranchs being almost 
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