446 GENERAL CHARACTERS OF ECHINODERMATA. ECHINIDA. 



globular shape of the Echinus gives place to a more prolonged 

 form, which, in some species, even approaches to that of the 

 Vermiform tribes (Fig. 609.). 



1001. The three genera just named may be taken as the 

 types of three Orders, into which this group may be conve- 

 niently divided. Between these orders, the links of transition 

 are so very gradual (being often afforded by fossil species when 

 no recent forms supply them), and the general similarity of 

 organisation with many important differences is so well marked, 

 that the whole group may be regarded as eminently natural. 

 These orders we shall now describe separately, commencing with 

 the one which may be regarded as most characteristic of the 

 class, that, namely, which contains the Echinus and its allied 

 forms. 



ORDER I. ECHINIDA. 



1002. In the order Echinida, the whole body is inclosed 

 with in a hard shell, usually of globular form, which is made up 

 of a number of polygonal plates. The structure of the shell is 

 uot analogous to that which is found among the Mollusca. 

 Instead of being solid, it is extremely porous ; and seems as if 

 it were made up of areolar tissue (ANIM. PHYSIOL. 24), 

 consolidated by carbonate of lime. The same kind of structure 

 is met with in the hard parts of the entire group. In order that 

 it may keep pace with the growth of the soft parts, it is neces- 

 sary that the shell should be capable of enlargement, or that it 

 should be periodically thrown off and renewed, like that of the 

 Crab. The former plan is here adopted ; and it is beautiful to 

 observe how perfectly the plan by which it is carried into effect 

 is adapted to the wants of this simply-organised animal. The 

 soft parts being, as it were, boxed up within the shell, it is 

 obvious that a mere addition to the edges which surround its 

 openings (such as takes place in the Mollusca), would rather com- 

 plete its inclosure than add to the interior space. But by the 

 regular addition of new matter to the edges of each plate, the 



