1 2 8 Introduction to A nimal 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. 18, b). 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 

 viviparous. In common with all Echinoderms, they 

 can reproduce lost parts. 



This class contains six orders: 



i. Blastoidea (Fleming) Palaeozoic, pentameral, anu! 

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

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

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

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

 unequal pentagonal basal tables; the second circle of live 

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

 radial plates. Above the calyx are five lancet-shaped ambu- 

 lacral pieces, homologous to the sub-ambulacral plates of 

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

 pinnulze along their margins ; their surfaces are striated, and 

 their margins perforated by pores. About fifty fossil spec: 

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

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

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

 single (Eleuthcrocrinus). 



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 pla: 

 in radial zones; mouth central, surrounded by pinnules or 

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



