168 
LECTURE XI* 
considered rather as forming a general outline 
than a minute and strictly accurate illustration of 
the subject. 
Among those genera whose inhabiting animal 
differs from the rest as to its nature, the genus 
De.ntalium is an example : the shell is shaped 
like an Elephant’s tusk in miniature, and its 
inhabiting animal is supposed to be allied to a 
Terebella. The genus Serpula is of various shape 
in the different species, but is generally of an ir- 
regularly twisted appearance, resembling a long 
tube warped in different directions. Its inhabit- 
ant is also supposed to resemble a Terebella, 
The genus Teredo is in reality a. kind of naked 
W'orm, which lines with a shelly matter the wind- 
ing or irregular cavities which it forms in Avood or 
other substances : its head is armed with a pair of 
very strong calcarious or shelly jaws, with which 
it works its way into the substance it inhabits, 
which is generally the wood of the bottoms of 
ships. This is the celebrated and destructive 
animal called the Ship-Worm, the Teredo navalis 
of Linmeus, so formidable for its ravages, and 
which hardly any contrivances yet suggested by 
human ingenuity have been found fully sufficient 
to prevent. Thus a contemptible worm, multiply- 
