AMONG THE SEA UKCHINS. 263 



Urchin, Echinus lividiis^ as it was formerly called, but the spines 

 have all been removed, the tubercles or bosses on which they 

 rotate alone appearing. 



Our next illustration gives a view of the shell cut open with 

 the spines still on. The tips of the teeth can just be seen below the 

 shell, meeting in a point, but the upper portion of the jaws can be 

 seen in situ, though not so well as in the illustration on p. 15, 

 Fig. C in our previous article. The jaws are composed of five 

 triangular pyramids (Fig. 3), through each of which runs a grooved, 

 pointed tooth ; these meet in the centre of the buccal mem- 

 brane, shown at Fig. 5, PL XVIII. The constant wearing away of 

 these calcareous teeth is provided for at the upper or basal 

 end, where the new growth forms, which is enamelled as it 

 descends lower down. The jaws are in muscular contact at their 

 sides, and from all parts of the dental pyramid a complicated set 

 of muscular bands, working through the " auricles," give to the 

 jaws great contractile power and capabiUty of motion and grip. 



In the illustration, Fig. 4, are to be seen the arch-like auricles^ 

 or " perignathic girdle," formed within the lower portions of the 

 shell proper, and intended for the support of the whole dental 

 apparatus, while through the arch-like openings proceed the five 

 radiating canals connected with the tube-feet. By cutting round 

 the peristomial margin of the shell, shown at / on p. 15, Fig. C, 

 with a penknife within the perignathic girdle, it is easy to 

 penetrate the skin of the buccal membrane, and take out the 

 entire mass, or " Aristotle's Lantern," as this singular structure 

 was formerly called ; for this membrane is only strengthened 

 by numerous small plates, magnified in Fig. 6, PI. XVI II,, though 

 these imbricated plates increase in number at the coronal edge of 

 the test. There are, however, on this membrane numerous pedi- 

 cellaria and tentacles, especially ten large sensitive ones, called 

 the buccal tentacles, which are protruded through the ten buccal 

 or actinal plates which surround the mouth, drawn, at Fig. 5, 

 PI. XVIII., from one of my own preparations. The tentacles and 

 much of the muscular flesh of the membrane was first dissolved 

 away in liquor potassae ; but what is left makes a polariscopic 

 mount of surpassing beauty. Around the margin of the mem- 

 brane, where it joins on to the coronal plates of the test, there 



