68 HOMES UNDER THE SEA. 



of rings edged with radiating spinelets. The common 

 " mare's tail " of our streams will give a good idea of the 

 shape of a Cake Urchin spine. 



The use of the spines is twofold. 



In the first place, they act as jointless legs, while tha 

 animal is crawling upon a tolerabty level surface, the 

 body rolling over and over as the spines are successively 

 raised and depressed. In the next, they act as digging 

 instruments, whereby the creature can sink itself into 

 the sand ; an operation which it performs with sur- 

 prising rapidity. 



Not only can they scoop holes in the sand, they can 

 even bore into hard rock. The Echinus, like the Limpet, 

 has a sort of home in which it usually lives. 



In the Crystal Palace, during the Electric Exhibition, 

 some singularly interesting specimens were shown of 

 objects that were brought to the surface while mending 

 marine cables. Among them was a block of limestone, 

 with five or six hemispherical holes large enough to 

 contain an ordinary orange, and in each hole was the 

 Echinus which had scooped it. 



The structure of these spines is singularly beautiful, 

 but cannot be properly seen unless a very thin transverse 

 section is taken, and placed under the microscope. It 

 will then be seen to bear a curious resemblance to the 

 corresponding section of any exogenous tree. The spine 

 of the porcupine presents a very similar appearance. 



Should the creature wish to climb a rock, the spines 

 would be useless. It therefore has to make use of the 



