EOHINODERMATA. 



161 



merable pieces accurately joined together, so as to form a globular 

 box enclosing the internal parts of the animal, but perforated at 

 each extremity of its axis by two large openings, one of which 

 represents the mouth, and the other the anus. 



The calcareous plates entering into the composition of this ex- 

 traordinary 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 recognisable in the figure by hemisphe- 

 rical 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 anal aperture being considerably the smallest, and 

 every succeeding plate becoming progressively larger as they ap- 

 proximate 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 lon- 

 gitude on a terrestrial globe, broad at the equator, but gradually 

 narrowing as it approaches the poles ; an arrangement, of course, 

 rendered necessary by the spherical form of the creature. There 

 are ten rows of these tuberculated plates ; but as they are disposed 

 in pairs, each row of large pieces being united by a zig-zag suture 

 with another of a similar description, there are in reality only five 

 large segments of the shell, each supporting a double row of 

 tubercles. 



The reader must not, however, conclude that the great central 

 tubercles above mentioned are the only parts of the shell to which 

 spines are affixed ; hundreds of smaller elevations are disseminated 

 over the surface, to which smaller spiculae are appended, although, 

 from their diminutive size, these are of secondary importance in 

 locomotion. 



The five large double segments which thus form the greater por- 

 tion of the calcareous shell are separated from each other by the 

 interposition of ten rows of perforated plates, likewise disposed in 

 pairs, and composed of much smaller pieces than those which sup- 

 port the tubercles ; hundreds of foramina, which pierce these ambu- 

 lacral bands, give passage to as many tubular feet or protrusible 

 suckers, in every respect resembling those of Asterias, and dis- 

 tended by a similar apparatus. 



It is impossible by any verbal description, at all commensurate 

 with the limits of our present undertaking, adequately to explain 



M 



