viii EGHINODERMATA MORPHOLOGY OF SKELETON 345 



3. Neither the ambulacra! nor the interambulacral plates are continued on to the 

 peristome (ffolcctypoida, Clypcastroida, Spcdanyoida). 



Among the Palccechinoidea also there are forms in which the perisomatic plates 

 reach as far as the mouth ; in Lepidocentrus, indeed, they do this in such a way as 

 to make it impossible to distinguish the coronal from the peristomal plates. 



Apart from the peristome plates just mentioned, the oral area is beset all over 

 with small irregularly arranged calcareous bodies. 



With regard to the number of coronal plates which border the peristome (mar- 

 ginal plates of the peristome), it is to be noted that in regular Echinoidea (Cidaroida, 

 Diadematoida), and even in most Holedypoida, ten pairs occur, five ambulacral and 

 five interambulacral. There are, however, certain Holectypoida in which, in one or 

 several interradii, only a single marginal plate occurs. In the Clypcastroida (Fig. 

 306) and Sjmtangoida (Fig. 302) the peristome is, as a rule, bordered by five pairs of 

 ambulacral and five single interambulacral marginal plates. Exceptions to this rule 

 are found in the Spatangoid division, the Cassiduloidea, where, for example, among 

 the Echinoneidce, EcJiinoneus and Amblypygus have two marginal plates in their 

 second and fourth interradii and only one in the others. 



(e) Manner in which the Skeletal Plates are Connected. 



In most Euechinoidea the plates of the skeleton, at least those of the corona, are 

 firmly and immovably connected together by means of sutures, and thus form a 

 rigid test. This is not the case in very many Palceechinoidea, and among the 



FIG. 305. Oral area of Cidaris papillata, Leske, from within (after Loven). 

 apo, Perignathous apophyses. 



Euechinoidea in the Diadematoid Echinotkuridcc ; also, as far as the skeleton of the 

 peristome is concerned, in the Cidaroida (Fig. 305). The edges of the plates here 

 overlap, i.e. they are imbricated. In the Echinothuridcc the plates are divided from 

 one another by strips of imcalcified connective tissue, which, to some extent, 

 allow the test to change its shape. The imbrication of the ambulacral plates is in 

 a direction opposite to that of the interambulacral. Viewing the test from without, 



