216 THE AQUARIAN NATURALIST. 



stud the exterior of the body, and we have indeed a 

 microcosm alike worthy the contemplation of the 

 philosopher and the admiration of the student. 

 Surely, exclaims an old writer, the hand that made a 

 Sea-urchin was well able to build up a world ! 



The calcareous plates entering into the composition 

 of this extraordinary shell may be divided into two 

 distinct sets, which differ materially in size as well as 

 in the uses to which they are subser vient. The larger 

 pieces are recognizable by hemispherical tubercles of 

 considerable size attached to their external surface, 

 adapted, as we shall afterwards see, to articulate with 

 the moveable locomotive spines. Each of these larger 

 plates has somewhat of a pentagonal form; those 

 which are situated in the neighbourhood of the mouth 

 and of the opposite aperture being considerably the 

 smallest, and every succeeding plate becoming pro- 

 gressively larger as they approximate the central 

 portion of the shell. The entire series of pieces in 

 each row resembles in figure the shape of the space 

 included between two of the lines which mark the 

 degrees of longitude on a terrestrial globe, broad at 

 the equator, but gradually narrowing as it approaches 

 the poles ; an arrangement of course rendered neces- 

 sary by the spherical form of the creature. 



The reader must not, however, conclude that the 

 large central tubercles above mentioned are the only 

 parts of the shell to which spines are affixed ; hun- 

 dreds of similar elevations are disseminated over the 

 surface, to which smaller spiculse are appended, al- 

 though, from their diminutive size, these are of 

 secondary importance in locomotion. 



