ECHINID^E. 145 



portion of the animal's body when it is very young, the 

 number of the other plates of the integument increasing 

 with age, and enlarging by means of a deposition of poly- 

 gonal calcareous particles at their edges. 



The Echinidse have been investigated by several very 

 distinguished naturalists, with a view to their classification, 

 especially M. de Blainville, Mr. Gray, M. Desmoulins, and 

 Professor Agassiz. While in most of the tribes of the 

 Echinodermata, the constituted genera, till lately, partook 

 rather of the character of families, in this order there has 

 been a tendency to an extreme multiplication of genera. 



The British species of Sea-Urchins belong to three fami- 

 lies, — 1 . The Cidarites, including Echinus and Cidaris. 

 The animals of this family have the mouth central and 

 inferior, the anus central and superior. The five ambulacra 

 form continuous rows from the apex to the mouth ; the 

 ovaries are five, and the mouth is furnished with a com- 

 plicated dental apparatus. The spines of these Sea-Urchins 

 vary in almost every species, and furnish good specific 

 characters ; but in the other families their form and sculp- 

 ture is generic, or rather characteristic of the whole group. 



2. The Clypeasteri^e, which have both mouth and anus 

 below, the former central, and usually armed with teeth. 

 Their ambulacra are not continuous, and there are five 

 ovaries. To this family belong Echinocyamus and Echino- 

 rachnim. 



3. The Spatangace^e. These have the mouth eccentric 

 and inferior, and the anus terminal. There are no teeth ; 

 the ambulacra are not continuous, and the ovaries are four 

 in number. To this family belong the genera Spatangus, 

 Brissus, and Amphidotus. 



