SEA UBCHINS MATEKIAL AND DEVELOPMENT. 75 



which passes between the plates and secretes the calcare- 

 ous matter of which they are composed. Additional 

 material is constantly added to the edges of the plates, ? o 

 that the growth of the entire structure must necessarily 

 be equable in every direction, and its shape preserved 

 unaltered. These hexagonal plates are slightly indicated 

 in Fig. 5 (page 71). 



A similar provision for equal growth may be seen in 

 the shell of a common tortoise, the concentric lines show- 

 ing the manner in which the additional material has been 

 deposited. 



On some of our coasts, and especially in the Channel 

 Islands, the Urchin is used as food. At the proper season 

 of the year, the interior is almost entirely filled with 

 eggs. These Urchins are boiled, and go by the popular 

 name of Sea Eggs. 



One of the most interesting points in the life history 

 of these creatures is the mode of their development. 

 When hatched from the egg, they bear as little resem- 

 blance to their perfect form as a caterpillar does to the 

 brilliant insect into which it will in time be changed. 



All these animals, whether Starfishes, or Urchins, or 

 Holothures, assume, when they first issue from the egg, 

 a form so totally unlike that of their parents, that only 

 of late years has their real status been known. The 

 young of the Starfishes and Urchins had long been known 

 to naturalists under the name of Pluteus, and are so simi- 

 lar in form that the description of one will answer very 

 fairly for the other. 



