152 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 ; 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 whidi 

 the creature fixes itself at pleasure to any object, 

 and stops its motion ; they are possessed of a very 



