INTERNAL STRUCTURE OF THE SEA-URCHIN. 



95 



muscles. The dentary apparatus, or lantern, is now ex- 

 posed, together with its complicated system of muscles, 

 occupying the centre of the oral half of the body cavity. 

 Before examining the muscles the hard parts should be 

 studied, and this may be done to best advantage with a 

 dried specimen. In such a specimen notice : — 



a. The points of the five teeth proti-uding from the 

 mouth. 



5. Cut with a sharp knife through the peristome, close 

 to the stony edge of the mouth pentagon ; lift up one edge 

 of the peristome and pull the lantern out of the shell. It 

 is made up of a number of calcareous pieces, which are 

 so aiTanged as to form a cone, with a broad, flat, circular 

 base (Fig. 41, «), which is turned 

 towards the anal surface ; and an 

 oral apex (Fig. 41, 5), made up of 

 the tips of the five teeth. The 

 axis of the cone is a tube for the 

 passage of the oesophagus. Pull 

 ofl* the peristome, and the remains 

 of any soft parts which ma}' adhere 

 to the lantern, and in a side view 

 notice : — 



Fig. 41. — Side view of the lantern. 

 (Drawn from nature by H. J. Rice.) 



a. Upper siu^ace of base. 6. Apex, 

 c. Alveoli. il. Teeth. e. Epiphyses. 

 /. RaduliB. 



1. The five triangular pieces or alveoli (Figs. 41 and 

 42, c), which carry the teeth. The peripheral surface of 

 each alveolus is nearly an isosceles triangle, with a short 

 base, and two much longer sides. The base is at the top, 

 the acute angles at the bottom, and the long sides of ad- 



