20 ORGANS OF SUPPORT, 



the columns. There are at an average at least ten tubercles 

 for each tubercular plate, and three tubercles for each of the 

 tuberculated portions (Fig. 8, 1, b, b,) of the ambulacra! plates. 

 Every tubercle of the shell supports an external, moveable, 

 calcareous spine ; so that there are more than ten thousand 

 pieces in the shell of the echinus esculentus, without counting 

 the complicated dental apparatus of the mouth, or the respira- 

 tory and ovarial plates, or the very minute calcareous pieces 

 disposed irregularly on the coriaceous membrane around the 

 oral and the anal orifices. There are five large heart-shaped 

 plates disposed around the anal aperture, as seen in the central 

 part of Fig. 7- B, each of which is perforated by a large round 

 hole for the termination of one of the five oviducts. Between 

 the tapering exterior ends of these five ovarial plates there are 

 five smaller heart-shaped plates, each of which is likewise per- 

 forated with a small round hole. Around the lower orifice of 

 the shell the last pairs of tubercular plates send in arched 

 processes which meet each other over the ambulacral plates, 

 nud u;ive a fixed and extensive surface for the muscles of the 

 dental apparatus. The five teeth and their alveoli are repre- 

 sented as seen laterally at Fig. 8, 2, and as seen from above at 

 Fig. 8. 3. There are five teeth, (Fig. 8, 2, 0, a,) of a compact 

 and dense texture where they project downwards from the 

 alveoli, and of a loose and fibrous structure at their upper 

 part, where they are enclosed in their complicated alveoli. 

 The alveoli (Fig. 8, 2, b, b,) are long, slightly curved, hollow, 

 tapering from above downwards, moveable individually and 

 collectively, and held in connection by several distinct move- 

 able pieces, (Fig. 8, 3, b, b y ) at their proximal extremities. This 

 dental apparatus, which exists also in the cidaris, is wanting 

 in the spatangus, and several other genera. 



The exterior surface of the shell is covered, in the echinida, 

 with solid calcareous spines, which rest and move upon 

 the round tubercles. These spines are very large in the 

 cidaris, where the shell is small ; and they are small in 

 the echinus and spatangus where the shell is large. They 

 grow like the other parts of the shell, by successive 

 deposition from the enveloping fleshy substance. Sec- 

 tions of these spines, and of the tubercles on which 

 they rest, are represented in Fig. 9, where 1, represents the 

 entire spine of the common echinus esculentus, and 2, a 



