COMMON EGG-URCHIN. - I 53 



ten, would give the great number of 8720 pores ; but as 

 each sucker occupies a pair of pores, the number of suckers 

 would be half that amount, or 18G0. The structure in 

 the Egg-Urchin is not less complicated in other parts. 

 There are above 300 plates of one kind, and nearly as 

 many of another, all dove-tailing together with the greatest 

 nicety and regularity, bearing on their surfaces above 4000 

 spines, each spine perfect in itself, and of a complicated 

 structure, and having a free movement on its socket. 

 Truly the skill of the Great Architect of Nature is not less 

 displayed in the construction of a Sea-Urchin than in the 

 building up of a world ! 



The whole of the external surface when the animal is 

 alive is invested with a fine membrane, as also the spines, 

 the joints of which have their flexibility greatly increased 

 by a thick fleshy envelope. It has been stated by Pro- 

 fessor Ehrenberg that the membrane of these spines is 

 clothed with vibratile cilia ; but such cilia I have never 

 been able to see, nor are any currents produced in the 

 water around the spines which would lead us to suspect 

 their presence. Arguing from analogy there should be no 

 such cilia. The mouth of the Urchin is placed in the 

 middle of a wide circular web of muscular skin which occu- 

 pies the concave and central part of the base. This skin 

 is studded with scattered calcareous tubercles. It is very 

 muscular. In the centre is seen the oral opening, and the 

 points of the five teeth projecting from it. There are ten 

 small fleshy tubercles placed on the margin, and at some 

 distance from it are seen ten tubular tentacula also placed 

 in pairs. On the oral margin of the shell, and between 

 the origins of the suctorial avenues, are seen ten brown 

 plumose appendages, somewhat resembling the tentacula 

 of Holothuria, of which I regard them as the analogues, 



