VIII 



ECHINODERMA TAS YSTEMA TIG RE VIE W 



313 



Family 3. Pentacrinidse. 



Calyx small as compared with the stem and the arms ; it consists of 5 basals 

 and 5 radials. (In the genus Extracrinus the infrabasals are separate). Rays 

 divided one to ten times. Stem surrounded at intervals by whorls of cirri. No 

 root-like processes on the stem. One or more free primary brachials. Orals wanting 

 in the adult. Trias, to Recent. Pentacrinus, Metacrinus (Fig. 257), Extracrinus, 

 Balanocrinus. 



Family 4. Comatulidae. 



Adult free, larva stalked. The calyx is closed apically by the uppermost ossicle 

 of the larval stem, which is fused with the larval infrabasals ; this ossicle carries cirri 

 and becomes detached from the rest of the stem. It is called " centrodorsal. " 

 The basals are externally visible, or else form an internal hidden rosette. Five or 

 ten simple or branched rays. The radials of the radial circle are usually followed, 

 in forms with divided arms, by two fixed primary brachials. Interradials wanting. 

 Orals wanting in the adult. Atdccrinus (basals externally visible), Eudiocrinus, 

 Antcdon (Fig. 258), Promachocrinus, Adinometra (the only Crinoid genus with 

 excentric mouth). Since Jurassic times, many living species. 



SUB-CLASS 2. Cystidea. 



Body (calyx) oviform or spherical, plated with numerous very variously shaped 

 pieces, which are rarely quite regularly, and often irregularly arranged ; stalked, sessile, 

 or (rarely) free. Arms in many cases unknown, perhaps wanting in many forms ; when 

 present, weakly developed, resembling pinnules, and rising near the mouth. Food 



FIG. 261. Orocystis Hehnhackeri, 

 Baur (after Barrande). 1-3, The FIG. 2(52. -Agelacrinus cincinnatensis. 



three apertures. 



grooves, arranged irregularly on the calyx, radiate from the mouth. At some dis- 

 tance from the mouth a second aperture (anal aperture), and between the two a third 

 aperture of unknown significance. Double pores or " pectinated rhombs " on some or 

 all of the plates. Palaeozoic Pclmatozoa, whose organisation is still little understood. 



Order 1. Cystocrinoidea (cf. the section on the perisomatic skeleton of the 

 Cystidea} : Porocrinus, Caryocrinus, Echinocncrinus, Cystoblastus (Fig. 259 A and B). 



Order 2. Eucystidea : Protocrinus (Fig. 260), Glyptosphccrites, Orocystis (Fig. 

 261), Echinosphccra, Aristocystis, Ascocystis, Mesites, Agelacrinus (Fig. 262). 



