CLASS I 



ECHINOIDEA 



263 



which serve in orienting a specimen. These madreporic pores are rarely 

 recognizable in Paleozoic Echini, and may have been wanting in some genera. 

 In the Exocycloida the posterior genital is usually imperforate or wanting, 

 and two or more to all genitals may be fused in a mass. Genital plates 

 may be in contact dorsally, forming a closed ring, or may be in part or wholly 

 separated by the oculars. 



Ocular plates dorsally cover the ambulacra and laterally the inter- 

 ambulacra in part on either side. Each ocular has a single pore. In 

 Paleozoic forms, oculars are apparently imperforate or rarely with two pores. 

 The pores are related to a primitive large tentacle and not to an ocular organ. 

 Ocular plates may all separate the genitals, reaching the periproct, when they 

 are described as insert, the usual Paleozoic character (Fig. 433, B) ; or they 

 may be all excluded from the periproct by the contact of the genitals, when 



B 



D 



Fio. 368. 



Typical ocular plate arraiigeiuent in regular Bcliini (after Jackson). A, Cklaris eoronata Goldfuss. Upper 

 Jura; Sontheim. All oculars exsert; plates sliaded. B, SaUnocidaris profundi (Duncan). Recent; Tristan 

 da Cunha. (Jcular I insert ; plates shaded. C, Acrosaleniaspinosa A^^vissvi. Cornbrash ; Chippenham, England. 

 Oculars I, V, insert. D, Centrechimis setosus (Leske). Recent ; Bermuda. Oculars I, V, IV, insert. 



they are described as exsert, the usual Mesozoic character (Fig. 368, A). 

 Oculars are all exsert in the young of probably all Recent and Mesozoic 

 regular Echini. In adults the same character may obtain, or one or more to 

 all oculars may travel in with development, separating the genitals so as to 

 be insert. As shown by Jackson, when oculars become insert, they do so in a 

 definite sequence in relation to the antero-posterior axis. The first ocular to 

 become insert is either I, or V. If ocular I comes in first, then V follows, or 

 the converse, thus marking the posterior pair or the bivium ; next ocular IV 

 becomes insert, then II, thus marking the posterior pair of the anterior 

 trivium ; lastly, if at all, ocular III becomes insert (shown in part in Fig. 368, 

 A-D). 



The apical disk is relatively large in very young Echini and in primitive 

 types {Bofhriocidaris, Cidaroida). It decreases rapidly proportionately in size 

 with growth, and is relatively small in specialised regular Echini [Echinometra, 

 Melonechinus, Lepidesthes, Fig. 434). 



In the Exocycloida the genital plates may be in contact at their sides, 

 forming a compact system (Fig. 369, D) ; or they may be separated by some of 

 the ocular plates which meet along the median line and separate the posterior 

 genitals, forming an elongate system (Fig. 369, C). When the two posterior 

 ambulacra (bivium) do not terminate at the summit in line with the other three 



