THE 



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remarkable appendages have been compared by some authors to the little hooks on the 

 calyx of the well-known burdock. 



The central figure represents a Synapta in its usual state, and the lowest figure ex- 

 hibits the same creature as it appears when it is annoyed and is explaining its feeling 

 by the simple process of cutting itself to pieces. Insufficiency of food will induce the 

 creature to perform this strange act, and impure' water will have the same effect. 



We now come to a new and beautiful family of this order, called Echinidae, because 

 they are covered with spines like the quills of the hedgehog. Popularly, they are known 

 by the name of SEA-URCHINS, or SEA-EGGS. 



In all these curious beings the upper parts are protected by a kind of shell always 

 more or less dome-shaped, but extremely variable in form, as will be seen in the following 

 illustrations. The shell is one of the most marvellous structures in the animal kingdom, 

 and the mechanical difficulties which are overcome in its formation are of no ordinary 

 kind. In the case of the common SEA-EGG, which is shown on the following illustra- 



Synapta digltata. (In health.) 



Cucumarla Hyallaa. 



Syoapta digitate. (Unwell.) 



tion, the shell is nearly globular. Now, this shell increases in size with the age of the 

 animal ; and how a hollow spherical shell can increase regularly in size, not materi- 

 ally altering its shape, is a problem of extreme difficulty. It is, however, solved in the 

 following manner : 



The shell is composed of a vast number of separate pieces whose junction is evident 

 when the interior of the shell is examined, but it is almost entirely hidden by the projec- 

 tions upon the outer surface. These pieces are of a hexagonal or pentagonal shape, 

 with a slight curve, and having mostly two opposite sides much longer than the others. 

 As the animal grows, fresh deposits of chalky matter are made upon the edges of each 

 plate, so that the plates increase regularly in size, still keeping their shape, and in 

 consequence the dimensions of the whole shell increase, while the globular shape is 

 preserved. 



If a fresh and perfect specimen be examined, the surface is seen to be covered with 

 short sharp spines set so thickly that the substance of the shell can hardly be seen through 

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