STRUCTURE OF TEST. 187 



this consists of a series of meridional areas composed ol 

 numerous small plates ; and we shall find that these areas 

 form ten alternating series, each of which consists of two 

 meridional rows of the aforesaid plates. The line of 

 division between the two rows in each area is well marked, 

 although the divisions between any two areas are much less 

 distinct. It will farther be seen that, while in one series 

 of areas (i a) the plates are much wider than in the other 

 series (a), they are also somewhat deeper. Now, if we 

 were to look from the inside of the test towards the light, 

 we should find that the outer half of each plate in the 

 narrower areas has several minute perforations ; and it is 

 through these minute perforations that, during life, the 

 animal protrudes the curious tube-like feet, by the sucker- 

 like action of which an urchin is enabled to climb up the 

 glass wall of an aquarium. Since these narrower areas are 

 connected with the function of progression, they are 

 appropriately termed the ambulacral areas, and the larger 

 intervening spaces are accordingly called the intermediate 

 areas. It is almost superfluous to add that the whole test 

 of an urchin thus consists of five ambulacral and five 

 intermediate areas, which between them comprise twenty 

 separate rows of plates, each running continuously from 

 one polar aperture to the other. During life, each plate is 

 separated from its neighbour by a thin membrane, and 

 the test increases in size both by 'additions to the edge of 

 each plate and also by the interpolation of fresh equatorial 

 zones of plates between the upper edge of the corona and 

 the apical disc. The spines of the urchins are movably 

 attached to the knobs with which the test is covered, and 

 vary much in form and size, although the limits of this 

 chapter do not admit of further reference to them. 



Reverting to Fig. 57, we may also see that the ambulacral 

 areas of the common urchin form a five-rayed star, on the 

 test of which, in the position of the figure, three rays are 

 turned away from the spectator and two towards him. 

 Now this radiate arrangement at once forcibly reminds us 

 of a star- fish, and any person who has ever handled those 

 creatures when alive will be aware that from their under 

 surface they can protrude tube-like sucking feet, precisely 



