GENERAL CHARACTERS OF ECHINODERMATA. ECHINIDA. 455 



1091. The three forms just named may be taken as the types 

 of three Orders, under which the majority of the species of this 

 group may be conveniently arranged. A fourth Order includes 

 the Hair-stars, of which few species at present exist, although 

 in former periods of the earth's history they were exceedingly 

 numerous ; and a fifth may serve for the reception of some 

 worm-like forms allied to the Holothuria, which evidently con- 

 stitute the transition from the Echinodermata to the Annelida, 

 with both of which classes they seem to have the most intimate 

 relations. 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, name- 

 ly, which contains the Echinus and its allied forms. 



ORDER I. ECHINIDA. 



1092. In the Order Echinida, the whole body is inclosed 

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

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

 not 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 



