THE ECHINOIDEA 



297 



3. The arbacioid, when the compound plates are formed of 

 three simple plates, the middle one being a large primary, while 

 the other two are small demi-plates (Fig. XII. 3). 



4. The echinoid, when the compound plates are formed of three 

 simple plates, but the middle plate is a small demi-plate, and the 

 two others are primaries (Fig. XII. 4). 



5. The cyphosomoid type, when the compound plates are formed 

 of many simple plates arranged in arcs, in which the middle com- 

 ponents are demi-plates (Fig. XII. 5). 



In most of the compound ambulacral plates, one or more of the 

 constituents become " demi-plates " by losing their contact with 

 the vertical suture on either side of the series. In some Echinoids 

 some of the plates are further reduced by growth-pressure, so that 

 they lie along the horizontal sutures between the primaries, as in 

 the Echinothuridae, or form broad areas of numerous small plates, 

 as in the Melonitidae. For these plates the name of " klasma- 

 plates " has been suggested. 



One important variation affects both the ambulacral and inter- 

 ambulacral plates. In some forms, such as Asthenosoma, the plates 

 are thin and attached to powerful lateral muscles (Fig. XXX.), by 

 which the test can be contracted and 

 expanded. In such Echinoids the plates 

 are not closely fitted like stones in a 

 mosaic, but the edges are bevelled, so 

 that the plates overlap like slates on a 

 roof. 



The Mouth Armature also under- 

 goes great changes, which may be best 

 seen by the nature of the perignathic 

 girdle, of which there are five main 

 types. In Cidaris (Fig. XIII. 1) it is 

 " disconnected," consisting only of an 

 erect "ridge" situated on the inter- 

 radial plates around the peristome. 

 In Salenia (Fig. XIII. 2), in addition to 

 the ridge, there are small " processes " 

 on either side of the ridge the pro- 

 cesses arise from the ambulacral plates. 

 In the Diadematidae and Echinidae 

 (Fig. XIII. 3) the ridge becomes insig- 

 nificant and the processes important ; 

 they lengthen and form an arch across 

 the ambulacra. The perignathic girdle 

 is then said to be " continuous." The extreme form is met with 

 in such genera as Echinometra (Fig. XIII. 4), where the arch is 

 strengthened by a strong cap. In those Echinoids in which the 



FIG. XIII. 



Types of perignathic girdles. 1, 

 disconnected type of Cidaris / 2, dis- 

 connected type of Salenia ; 3, simple 

 arch of Diadema; 4, capped arch of 

 Echinmnetra. a, ambulacral plates ; 

 our, auricle ; c, cap ; i, interainbula- 

 cral plates ; i.a.p, interambulaeral 

 processes. 



