vni EOHINODERMATA MORPHOLOGY OF SKELETON 339 



The whole perisome, from the mouth to the apical system, falls 

 into two sections : (1) a small portion surrounding the mouth, the 

 peristome or oral area ; and (2) the larger remaining portion be- 

 tween the peristome and the apical system, the corona. In the peri- 

 stome the skeletal pieces are usually loosely embedded near one 

 another, or imbricate one with the other, remaining movable one 

 against the other. Sometimes the peristome is membranous, without 

 skeletal pieces. In the corona the skeletal pieces are usually firmly 

 connected with one another by means of sutures, like the plates of 

 the apical system, together with which they form a rigid test. In 

 dead Echinoidea, and in nearly all fossil forms, this test remains 

 intact, while the skeleton of the peristome falls to pieces, and is 

 therefore rarely preserved. 



The perisomatic skeleton in all Echinoidea consists of two systems 

 of plates, which run from the apical system over the ambitus to the 

 mouth as ten meridional zones : five of these zones or systems of 

 plates are placed radially, and these are called the ambulacra. These 

 five zones, on which the tube-feet rise, are always in contact with the 

 five radial (ocular) plates of the apical system, so that each ambulacrum 

 touches an ocular plate with its apical end. The ambulacral plates 

 are perforated for the passage of the ambulacral vessels, which serve 

 for swelling the tube-feet. The five other zones or systems of plates 

 are interradially placed, and are called interambulaera or interambu- 

 lacral plate systems. They alternate regularly with the ambulacra. 



Considering the perisomatic skeleton of the Echinoidea more closely, the follow- 

 ing special points are worth attention. 



(a) The Number of the Vertical or Meridional Rows of Plates in the Ambulacra 

 (radii) and Interambulaera (interradii). 



In all JZuechinoidca (from Devonian times up to the present), the corona consists 

 of twenty meridional rows of plates, ten of which united in pairs belong to the 

 ambulacral system, and ten also in pairs to the interambulacral system. Five 

 double rows of ambulacral plates thus regularly alternate with five double rows of 

 interambulacral plates. 



In the exclusively Palaeozoic Palceechinoidea, the number of meridional rows of 

 plates in both ambulacra and interambulaera varies. The number of rows in all 

 the five ambulacra and in all the five interambulaera of individuals of one and the 

 same species is, however, always the same. 



In the ambulacra, however, the number of rows of plates in the Palseechinoidea 

 is usually two. The Melonitidce, (Fig. 301) form the only exception, having four to 

 ten rows in each ambulacrum. 



In the interradii, on the other hand, the number of rows of plates varies. 

 Bothriocidaris has only one single row of plates in each interradius. In all other 

 Palceechinoidea there are more than two (3-11) rows of plates in each interradius 

 (Fig. 230, p. 289). The interesting genus Tiarechinus (Fig. 231, p. 289) is dis- 

 tinguished by the great simplicity of its interradial system of plates ; in each inter- 

 radius there are only four plates, a single one at the edge of the peristome the 

 large marginal plate of the peristome and three intercalated between this and the 



