SEA-URCHINS. 75 



from the feet, and they can then be withdrawn in 

 their soft and flaccid condition into the shell. 



It is curious to observe how the organs and parts 

 of the sea-urchin, starfish, and sea-cucumber bodies 

 are arranged in fives. Look at this dried shell of the 

 Echinus. You see it is beautifully built up of six- 

 sided plates of lime, firmly united to one another, and 

 arranged in ten zones, running from top to bottom of 

 the shell. Five of these zones are perforated with 

 holes for the emission of the tube-feet ; the plates of 

 the intervening five zones being imperforate. Now, 

 if you anatomise the sea-urchin's body, you will find 

 therein the same law of number regulating its structure. 

 There are five chief nerve- cords, and five eyes at the 

 top of the shell ; there are five tubes carrying water 

 to the feet, and there are five main blood-vessels. The 

 starfishes show the same number of parts, and so do 

 the sea-cucumbers. 



If we were dealing with plants, we should call this 

 phase of matters their " symmetry," for you find that 

 a law of number is also represented in flowers and their 

 parts. The primrose has five sepals, five petals, five 

 stamens, and five carpels : and your buttercup has the 

 same number of parts in sepals and petals, while its 

 stamens and carpels are present in multiples of five. 

 The lilies, tulips, crocuses, and snowdrops, on the 

 other hand, have their parts developed in threes or 

 in multiples of three. So that we observe how the 

 arithmetic of living Nature is not represented by mere 

 chance in her modelling work ; and in the starfish 

 tribe it is very clear, instead of considering " number 

 one," as the phrase runs, she may be said to have 

 devoted all her energies to " number five." 



The sea-urchin has a very ancient history as a 



