SEA-URCHINS, STARFISH, AND BRITTLE-STARS. 135 



good number of the empty shells, or tests, which usually 

 ornament cottage windows near the sea, and are to be found 

 on the beach at most seasons of the year. In addition, an 

 attempt should be made to obtain one or two living 

 specimens. It is not always easy to obtain the common 

 urchin (Echinus esculentus) in the living condition, but the 

 small purple-tipped urchin (E. miliaris) may generally be 

 found in the Laminarian zone, and has the advantage that 

 one may keep it alive in confinement longer than its relative, 

 which needs a great bulk of water. 



Let us examine the living specimens first. The common 

 urchin is really an inhabitant of fairly deep water, but I 

 have often taken single 'specimens at low spring tides, and 

 where the shore slopes steeply the urchins may sometimes 

 be seen in numbers by looking over the edge of the rocks. 

 The colour is usually purplish pink, but I have found 

 specimens entirely straw coloured, with beautiful purple 

 tube-feet. The test is rounded, and in life covered by 

 numerous long spines. In E. miliaris, which is very much 

 smaller, the diameter often not exceeding that of a penny, 

 the test is flattened, and the numerous spines are short and 

 not of uniform size. The general tint is green, but the 

 spines are tipped with purple. In either urchin you will 

 notice the mouth in the middle of the under surface. It is 

 surrounded by a membrane which is very extensile, so that 

 the mouth can be protruded to a considerable extent, and 

 then withdrawn. The object of this, as a living active 

 urchin will show, is to allow of the free movement of a 

 complicated tooth-bearing structure called Aristotle's lantern. 

 This contains a circle of five chisel-edged teeth (see Fig. 44) 

 which may be seen and felt in the mouth of the urchin, and 

 are borne by an arrangement of ossicles, which permit the 

 teeth to open and close so that the urchin can crop seaweed 

 as effectually as a rabbit crops dandelions. Their action is 

 greatly aided by the elastic mouth membrane, which is 

 covered by small tube-feet which act as tentacles, and by 

 little stalked forceps called pedicellarias, curious structures 

 common among the Echinoderms, and probably serving to 

 keep the test clean. 



The presence of this mouth-membrane and of Aristotle's 

 lantern has a rather interesting effect in the case of dried 



