344 On tJic Echinoidea or Sea- Urchins. [Sess. 



shell or box is found to be perfectly water-tight, having stood 

 the test of a hydraulic pressure brought to bear on it which 

 only stopped short of the bursting point, yet without showing 

 the slightest leakage. At the Eoyal Botanic Garden of our 

 city the experiment has been made of utilising the shells, by 

 turning them into flower-pots for hanging-plants ; but in a 

 comparatively short time these improvised pots always fell to 

 pieces, separating at the joints of the plates. In the English 

 translation of a popular scientific work entitled ' The World of 

 the Sea,' by the French naturalist M. Moquin Tandon, the 

 astounding statement is made that in the shell of the edible 

 sea-urchin {PsammecMnus (E.) csculcntiis), a near ally of the 

 common egg-urchin, there are 10,000 plates. One can only 

 make the charitable suggestion that in this case a " nought " 

 has been added by mistake, otherwise the statement is simply 

 absurd. I have, however, counted as many as 800 plates in 

 the shell of a well-grown egg-urchin. 



A glance at the test of an echinus further reveals the fact 

 that it is arranged in meridional zones or rays, thus evidencing 

 its affinity with the star-fishes, — with which animals there 

 are, indeed, other points of resemblance or identity, to be after- 

 wards noted. These zones are composed of twenty single 

 rows of plates, or ten double rows — five of the double rows 

 being made up of large plates and five of small plates, so that 

 the zones are of unequal size. The large and small zones 

 occur alternately, while the latter are perforated at their 

 margins by miniite openings or pores. These pores in the 

 small rows of plates are for the emission of the tube-feet, or 

 ambulacra, which the animal has the power of protruding and 

 retracting at pleasure. The small zones are therefore known 

 as the " ambulacral areas," while the alternating larger zones 

 are styled the " interambulacral areas." By looking through 

 the empty shell of a sea-urchin from the outside at the oral 

 aperture, and towards the light, these areas and perforations 

 are easily distinguishable. In the common egg-urchin there 

 are ihrcc pairs of pores in each row, each pair carrying one 

 tube-foot ; while in the edible sea-urchin of the Mediterranean, 

 four pairs of pores are present in each row. There are other 

 points of distinction between these two species, yet they are 

 frequently confounded ; and I notice that in one of the " price- 



