THE SANDY CREEPLET. 
299 
and compared them, without any other information, nothing 
would he more manifest than that we must assign them not 
only to distinct species, hut even to distinet genera. IMr. 
Alder has favoured me with many specimens, obtained hy 
^Ir. Barlee, at Shetland, some of which, each consisting of 
several full-grown polypes, are perfectly independent and 
compact, showing not the slightest trace of adhesion to any 
foreign body, nor of any part that can he distinguished as 
a root-hand. Thus, in the specimen figured in Plate ix. 
fig. 9, three polypes diverge from a common centre ; others 
are similarly formed, sometimes with a triangular dilatation 
of the point of divergence, which thus becomes flat, hut 
still with both surfaces eq^ually entire. 1 have not seen 
more than three polypes on any free specimen. 
But among these, we see specimens at first sight hardly 
distinguishable from them, except hy a sliglit globosity at 
the point of divergence : when we turn these over, we dis- 
cover that the globosity has been moulded on a minute 
shell, evidently that of a Natica. Then others occur, in 
which the shell, almost always a Natica, is larger, and 
there is a distinet basal carpet uniformly spread over it, 
of the sand-covered flesh, from which spring four or more 
polypes : these are manifestly identical with the free ones. 
But on larger shells the colony of polypes is made up of 
more individuals ; in one specimen before me, in which the 
shell is about the size of Natica Alderi, there are nineteen 
polypes. In every case the basal carpet has spread in 
uniform thickness over the entire shell, following the form 
aecurately, and extending to the edge of the outer lip, and 
clothing the rotundity of the inner lip as far as the eye can 
follow it. Strange to say, in every example, the shell itselt 
has wholly disappeared, and all that is left is the exact 
model of it in the sand-clothed membrane, or basal carpet, 
of the polype. 
