128 Iiitroductioii to Animal Morphology. 



Polian vesicles, and radial ambulacral canals with 

 pedicelli. Each of the last is a muscular tube, ending 

 externally in a foot and sucking disc, and having at 

 its inner or attached end a canal for the ambulacral 

 vessel, and an opening into a muscular ampulla or 

 reservoir (Fig. i8, h). The foot consists of longitu- 

 dinal and circular muscular fibre, covered by ciliated 

 epithelium. Contraction of the ampulla and of the- 

 circular fibres extends the foot, which is contracted 

 and retracted by the longitudinal fibres. 



The sexual organs are grape-like pouches in the 

 inter-radial spaces. Development is rarely direct, or 

 \'iviparous. In common with all Echinoderms, they 

 can reproduce lost parts. 



This class contains six orders : — 



I. Blastoidea {Fletning) — Palaeozoic, pentameral, armless,, 

 bud-like forms, fixed on a jointed, perforated, immovable 

 stalk, with a central mouth, inter-radial genital pores, and 

 with or without (Eleutherocrinus) an anus. The body (calyx) 

 consists of thirteen plates in three circlets, the lowest of three 

 unequal pentagonal basal tables ; the second circle of five 

 forked radial plates, between which are five trapezoidal, inter- 

 radial plates. Above the calyx are five lancet-sha})ed ambu- 

 lacral pieces, homologous to the sub-ambulacral plates of 

 crinoids, fitting into the forks of the radial plates, and bearing 

 pinnulae along their margins ; their surfaces are striated, and 

 their margins perforated by pores. About fifty fossil species 

 are known. They have large genital pores in five pairs, each 

 over an inter-radial (Pentatremites), or over an ambulacral 

 plate (Eloeacrinus). The pores maybe in three pairs and two 

 single (Eleutherocrinus). 



2. Cystoidea — Palaeozoic, pedicled, or sessile; body glo- 

 bular or polyhedral, rarely flattened (Agelacrinus), with a wall 

 of many (often 1-2-300), sometimes porous, polygonal plates 

 in radial zones ; mouth central, surrounded by pinnules or 

 weak arms, which may be fixed, or (rarely) free, pinnulated 



