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NAKED-GILLED MOLLUSKS. 105 
in seizing and securing its prey. It is covered 
with numerous transverse plates, armed with spines 
or teeth directed backwards. These teeth differ 
in number and in form; those of Holis papillosa, 
figured above, are very numerous and minute, 
being not more than one-sixth part of the thickness 
of a human hair. Their arrangement is in trans- 
verse arched rows, but in E. coronata, there is one 
large central tooth on each plate with denticulated 
sides; and in #, alba, a central tooth only, without 
denticulations. 
FAMILY DORIDID&. 
The members of this extensive family are cha- 
racterised by having the anal orifice placed in the 
medial line of the back, towards the hinder part, 
and the respiratory organs arranged around it in 
a more or less complete circle of leaf-lke plumes, 
so as to resemble the petals of a beautiful flower. 
The mouth is a small conical proboscis placed under 
the front edge of the mantle, where this latter 
organ is distinct, and sometimes guarded by two 
simple tentacles ; another pair of tentacles is placed 
on the front part of the back. These are peculiar 
in their structure, being beset with numerous 
narrow plates, arranged diagonally, parallel to each 
other. These tentacles, for the most part, are capa- 
ble of being withdrawn into proper cavities, the 
edges of which, in many cases, are raised into 
tubular sheaths to protect them. 
The Doridide are found on the shores of every 
sea. Many species are common on our own coasts, 
crawling on sea-weeds, or concealed beneath stones 
between tide-marks; some kinds, however, confine 
