242 EVENINGS AT THE MICROSCOPE. 



permanent joints with swollen condyles, or rounded heads, 

 resembling those of the antennas of a beetle. Hence the 

 Skeleton has great freedom and precision of motion ; 

 using the tips of the long toes as a fixed point, it throws 

 its body hither and thither to a great distance, with re- 

 markable agility. These joints admit of forward and 

 lateral flexure, but you never see the body brought back- 

 ward beyond a perpendicular position, the swelling of 

 the terminal portion of each articulation precluding 

 further motion in that direction; just as the joints of 

 our knees and elbows permit bending in one direction, 

 but not in the other. 



This is another indication that these divisions are true 

 joints ; and I direct your attention to the point, because 

 the fact helps to indicate that this class of animals has 

 its proper affinities with the Articulata, which has 

 been denied by most naturalists. 



The form is curious. Elevated at the summit of a long 

 foot, consisting of three joints, which surmount two un- 

 usually lengthened and slender toes, is a vase-shaped 

 lorica, which is three-sided. Its surface is covered all 

 over with minute points, very closely set, so that it re- 

 sembles shagreen ; besides which it forms numerous sharp 

 ridges, which run across transversely. The two sides run 

 off into thin lateral wings, which come to a sharp edge ; 

 the back angle also forms a ridge, but less sharp and thin. 

 In front, the shell, or lorica, is as it were cut off abruptly, 

 like the rim of a goblet ; but out of this rises a second 

 column, connected with the rim by an elastic membrane, 

 which allows some freedom of motion. This column is 

 widely divided in front and behind, and rises to a point 

 on each side. When the rotatory front is withdrawn, 

 these points approach and meet, closing the orifice ; but 

 when the head is protruded they are widely separated. 



Internally, we see the usual viscera contained in so 

 narrow a cavity that we are ready to suppose the walls of 



