44 INVERTEBKATE ZOOLOGY. 



niotitli is a crown of fringed retractile 

 tentacles. Anibnlacral feet, either in 

 rows, or irregularly disposed, or wanting. 

 Larval form the Auricidnria — reduced in 

 some cases. 



Order 1. Pedata Ambiilacral feet present. 



■^ Pen facta, Thyone. 



Order 3. Apoda Ambiilacral feet wanting. -^ Synajita. 



Class II. Crinoidea Mostly sessile, flower-like forms; with 



stem, calyx and many-branching arms 

 covered with pinniilae. Very few living 

 forms — many fossil. Free-swimming 

 larval form oval, with ciliated bands. 

 ■jf Pentacrinus. 



[The following fossil classes belong here. They are often included among Crinoids : 



Class. Cystidea Mostly with arms, calyx plates irregular. 



Qlass. Blastoidea Without arms, calyx plates regular.] 



Class III. AsTEROiDEA Adult star-shaped to pentagonal, with 



exo-skeleton in the form of a rough net- 

 work, stiidded with fixed spines. Ambu- 

 lacral feet in grooves on oval side. 



Order 1 . Asteridea No clear distinction between mouth and 



arms. Larval form the Bipinnaria and 

 Brachiolaria. -^ Asteria.'^. 



Order 2. Ophiuridea Disc and arms distinct, the latter serpen- 

 tine and very brittle. Larval form a 

 Pluteus, similar to that of Echinoidea. 

 ■^ OpMopolis. 



Class IV. Echinoidea Adult spheroidal, oval or disc-shaped, 



with exo-skeleton composed of solid cal- 

 careous plates arranged in meridional or 

 radial rows. The siirface of this shell is 

 beset with sjiines which rotate upon tu- 

 bercles. Rows of ambulacral feet pro- 

 ject from foramina in shell. 



