164 



HALF AN HOUR WITH SEA-UECHINS 



Fig. 87. 



produced annually, so that this concentric structure 

 is not observable. But, as seen in Fig. 86, there are 

 certain solid ribs and bands of open calcareous net- 

 work, the latter coloured purple, which chequer the 

 surface. 



Whilst speaking of the spines of sea-urchins, we 

 may notice certain minute and curious bodies which 

 occur both on them and on the spines of some of the 

 star-fishes, which were formerly supposed to be 

 separate parasitic animals. They are called Pedi- 

 cellarite, and are now considered as simple appendages 

 to the spines, although their true functions have not 

 yet been clearly made out. Each of 

 them consists of a thin stalk, formed 

 of carbonate- of lime, which is sur- 

 mounted by a curious, pincer-like 

 apparatus (Fig. 87). The whole is 

 invested with the general animal 

 membrane of the sea-urchin. The 

 pincers are double, and are formed 

 of a fine, calcareous net-work, resem- 

 bling that of the shell. The edges 

 of each limb of the pincers are ser- 

 rated, and the whole structure is 

 constantly engaged, in a ludicrous 

 snapping. Besides these peculiar 

 objects for the microscope, obtainable 

 from sea-urchins, we have others 

 known as spicules, which occur in various parts of 

 the body. In our most abundant species, the common 



Pedicellana, X 60. 



