ECHINOIDEA. 



105 



In one great group of the Echinoids, the ambulacral areas 

 pass from the centre of the base of the shell to its summit 

 (fig. 58;, when they are said to be " perfect" (ambulacra per- 

 fecta} or " simple." In another great group the ambulacral 



Fig. 58. Gulerites albogalerus. The first figure shows the under sun'ace with the 

 mouth and anus. The middle figure is a side view : and the right-hand figure shows 

 the upper surface, with the ambulacral areas converging to the apical disc. White 

 Chalk. 



areas are not thus continuous from pole to pole, but simply 

 form a kind of rosette upon the upper surface of the shell 

 (fig. 59). In these cases as in the common Heart-urchins 



I'ig- 59- Scutella subrotunda, showing petaloid ambulacra. Miocene. 



the ambulacral zones are said to be " circumscript" (ambulacra 

 circumscripta) or "petaloid." 



The most important external structures of the corona are 

 the tubercles and spines. The tubercles are rounded eleva- 

 tions upon which the spines are carried (fig. 60). They vary 

 much in their dimensions, and receive special names accord- 

 ing to 'their size or position on the test. Ordinarily the tuber- 

 cle consists of a rounded ball or hemisphere (the " mamelon ") 

 supported upon a conical process (the "boss") which arises 

 from the plate. The ball of the tubercle may or may not be 

 perforated for the insertion of a ligament which is attached to 

 the articular surface "of the spine. In many cases (as in fig. 

 60) the base of the tubercle is surrounded a round or oval 

 smooth and excavated space which is termed the " areola." 

 The spines are movable appendages which are jointed 



