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-ui chin, we find the whole surface of the 

 <hell 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 

 enables the jaws to play up and 

 down, and across, so that a more effective grinding-mill can 

 scarcely be imagined. 



The HolothuricB, 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- 

 sion and contraction of their 

 bodies. The mouth is surrounded by plumose tentacula, 



Dental Apparatus of the Sea-urchin, 

 viewed from above. 



A, Two sockets with teeth, of Echinus 

 eseulentus. B. Single socket with its 

 tooth viewed on. the outside. 



the 



