THE ORIGIN OF MAN 29 



of twenty columns of closely adjoining plates ar- 

 ranged in pairs. Five pairs of these are the am- 

 bulacral columns which are perforated by tube- 

 feet and alternate with five other pairs which are 

 not perforated, the interambulacral columns. 

 The mouth is on the lower or flatter side and is 

 often provided with a powerful jaw of a very 

 complicated structure. At the top of the dome 

 or corona is the anal opening, around which are 

 arranged ten plates in one or two circles. The 

 five large plates of the inner ring are the genitals 

 because each is pierced by an opening, the ter- 

 minus of the genital organs. The five smaller 

 plates of the outer ring, situated at the termini of 

 the five pairs of ambulacral columns, are known 

 as the oculars because in them are located the so- 

 called eyes. 



The crinids are Echinodermata which are usu- 

 ally gregarious in habit and fixed to the sea bot- 

 tom by a more or less long stalk, but there are, 

 however, many free forms that crawl or swim 



