231 



CHAP. XV. 



BATED ANIMALS SEA-URCHINS. 



Antiquity of the Eace. Egg-urchin. Complexity of Struc- 

 ture. Method of Enlargement. Mechanism of Spines, of 

 Mouth, &c. 



IT has been already stated that the fourth 

 family of the Echinodermata is that of the Echi- 

 nidse or sea-urchins. Although this creature 

 differs in appearance so completely from the 

 star-fish, an investigation of its structure places 

 it unquestionably in the same order. 



The urchin belongs to a race whose pedigree 

 extends far into the ages of hoar antiquity, hav- 

 ing existed thousands of years before man became 

 a denizen of this terraqueous globe. The species 

 now existing indeed are not found in a fossil 

 state, save in very recent deposits; but their 

 ancestors flourished in prodigious multitudes 

 during the secondary and tertiary epochs, and 

 are found imbedded in the oolite and chalk for- 

 mations, some shaped like helmets, some elliptical 

 in form, some turbinated, and others heart-shaped 

 like those of the present day. The urchin is 

 therefore an object of interest to the geologist 

 as well as to the student of natural history. 



