IOO BY THE DEEP SEA. 



urchin (Echinus mtliaris), which is a well-known inhabitant 01 

 rock-pools. It is enclosed in a stone box, which is a miracle 

 of design, for although there is no elasticity about it, and it 

 cannot be stretched, it yet serves the growing Urchin for years, 

 and never cramps hirn. There is never any necessity for 

 throwing it off, as the crabs and lobsters have to do repeatedly 

 with their suits of armour. The nearest parallel to it in nature 

 is the human skull, which although consisting only of a few 

 pieces, enlarges in a similar manner to accommodate the 

 growing brain. 



It is remarkable how, in the whole sub-kingdom Echino- 

 dermata, all the wonderful variety displayed by the many 

 species is found compatible with rigid loyalty to the dominat- 

 ing "number" principle: in these animals everything is 

 governed by the number five. With a few exceptions, the 

 rays are in fives, or multiples of that number ; so are the jaws, 

 the boundaries of the plates, and other details, as may be seen 

 in any of the Stars to which we have alluded. 



In the Sea-urchins we get an advance in that direction, for 

 its stone box is built up of nearly six hundred five-sided* plates 

 of lime, securely attached to each other by their edges, and 

 fitting with such beautiful accuracy, that there is not the ghost 

 of a crevice from base to crown of this wonderful cupola. 



But if there are no crevices there are many apertures 

 over five thousand of them in a full-grown Echinus escnlentus. 

 Forbes, many years ago, calculated there were 3,720 in a 

 moderate-sized specimen ; and his figures, though used in all 

 the books since his day, do not appear to have been checked. 

 But I have counted the pores in what I should describe as a 

 moderate-sized individual, i.e., one that measured, when de- 

 nuded of its spines, twelve inches in circumference, and find 



* Dr. Andrew Wilson, in his " Glimpses of Nature," impresses upon his readers 

 this "pentarchy" in the building of an Echinus, but curiously describes the plates a< 

 being ,s7.r-sided. He evidently had none but living specimens to refer to when he 

 thus wrote. It is only in a specimen from which the spines and skin have been carQ- 

 fully cleaned that the form of the plates can be seen. 



