153 
LECTURE X. 
times measures four inches or more in diameter* 
The body, or soft part within the shell, is mark- 
ed, as it were, into a kind of lobes or divisions, not 
much unlike those of the pulp of an orange ; the 
intestines are disposed in a somewhat circular di- 
rection, and the whole body is internally support- 
ed by a set of upright bony columns. On the 
outside of the shell, which is generally of a dull 
violet-colour, and sometimes greenish, are seated 
a prodigious number of sharp, moveable spines, 
curiously articulated with the tubercles of the sur- 
face, and connected by strong ligaments. These 
spines are the instruments of motion, by the assist- 
ance of which the animal conveys itself at plea- 
sure to any particular spot j and so tenacious are 
they of the vital principle, that, on breaking the 
shell, the several fragments have been sometimes 
seen to walk off in different directions. Between 
the spines, disposed in continued longitudinal 
rows or series, on the different divisions of the 
shell, are an infinite number of small holes, com- 
municating with tentacula or feelers placed above 
them : these feelers are the instruments by which 
the creature fixes itself at pleasure to any object, 
and stops its motion : they are possessed of a very 
