264 



EGHINODERMATA— ECHINOZOA 



PHYLUM IV 



(trivium), and are surmounted by oculars placed far posteriorly, the System is 

 Said to be disjimä or disconneded. The posterior oculars are then separated 



from the postero- 

 ' ^' ^ ^ lateral genitals by 



a number of inter- 

 ambulacral plates 

 intercalated along 

 the dorsum (Fig. 

 369, A). 



In the Clypea- 

 strina and many 

 of the Cassidulidae, 

 the apical System 

 consists of five 

 minute ocular 

 plates, and one 

 large, pentagonal, 

 central plate, which 

 probably represents 

 the fused genitals 

 and is usually 

 pierced in four or 

 in all of its inter- 

 radial angles by 

 genital pores (Fig. 

 369, E, F). 

 Periproä. — This structure which bears the anus is within the apical disk 

 in all regulär Echini, when the test is termed endocyclic ; and at a variable 

 distance beyond it in the median line of the posterior interambulacrum in 

 irregulär Echini, when the test is termed exocydic. Periproctal plates are 

 rarely preserved in fossil Echini. They are numerous, angular, thick, and 

 fill the area in Paleozoic genera and the Cidaroida ; they are numerous, or 

 few and dissociated, often reduced to granules in other Echini ; or the 

 periproct may be largely leathery (CentrecMnus). In the Saleniidae there is a 

 large suranal (Fig. 368, B), with additional large plates (some Acrosalenias) or 

 with small plates only. A suranal occurs in the young of some types as the 

 Echinidae and Strongylocentrotidae. The periproct is usually circular, but may 

 be angular, or in the Exocycloida varies from circular to elliptical or pyriform. 

 The Position of the periproct in the test is of great systematic importance. 



Peristome. — This is actinal and central in regulär Echini, and is circular, 

 decagonal or pentagonal in outline. Along its margin in the basicoronal 

 interambulacral plates of the corona there are ten incisions for the extension 

 of the peristomal gills which exist in certain Echini, the Centrechinoida and 

 Holectypina. In the Exocycloida the peristome is variable in shape and 

 Position, but it is actinal and is central or more or less anterior from the 

 centre. The oral membrane of the peristome is attached to the lantern when 

 present, otherwise the inner edge is free. 



The peristome may be more or less extensively plated or may be naked, 

 and the character of the plates is of systematic importance. In the young of 

 probably all regulär Echini there is one row of primordial ambulacral plates 



Apical Systems of irregulär Sea-iirchiiis. A, CoUyrites. B, Holectypua. 

 C, Hyboclypeus. D, Micraster. E, Conodypeus. F, Clypcaster. 



