THE SEA-URCHIN’S SHELL. 339 
whereas a complicated shell, such as the sea-urchin possesses, 
can grow in the same ratio as the internal parts, by continual 
deposits on the edges of the individual pieces. On closely 
examining a living sea-urchin, we find the whole surface of the 
shell and spines covered with a delicate skin, which, in spite 
of their close connexion, penetrates into the intervals of the 
several pieces. This membrane secretes the chalk of which the 
shell is composed, and deposits fresh layers on the edges of the 
plates, so that in this manner the shell continually widens until 
the animal has attained its perfect size. The spines are secreted 
in the same manner, and show under the microscope an admi- 
rable beauty and regularity of structure. So bountifully has 
the great Architect of worlds 
provided for the poor insignifi- 
cant sea-urchin ! 
The dental apparatus of the 
animal —the so-called lantern 
of Aristotle—is another master- 
piece in its way. Fancy five 
triangular bones or jaws, each 
provided with a long, project- 
ing, movable tooth. A com- 
plicated muscular system sets 
the whole machinery going, and  pental Apparatus of the Sea-urchin, 
enables the jaws to play up and Met Garvomy aD OF ee 
down, and across, so that a more effective grinding-mill can 
scarcely be imagined. 
The Holothuriaw, or Sea- 
cucumbers, may be regarded 
in one light as soft sea-urchins, 
and in another as approxi- 
mating to the Annelides or 
worms. Their suckers are 
similar to those of the true 
star-fishes and sea-urchins. 
Besides progression by means 
of these organs, they move, 
like annelides, by the exten- Boe sea eh the ee Fee el 
sion and contraction of their 
bodies, The mouth is surrounded by plumose tentacula, the 
