190 



ZOOLOGY. 



each side of which are attached a row of pinnules. Be- 

 sides Pentremites are the typical genera Elceacrinus and 

 Eleatherocrinus. 



Order 3. Cystidecs. This group is likewise extinct. In 

 the fossil Pseitdr>rri)iux there is a short-jointed stalk, while 

 in Caryocystites (Fig. 132) there is no stalk and no arms, the 



Fig. 132. -Caryocys- 

 tites, a Cyetidean. 

 After Liitken. 



Fig. 134. Agelacrintts, a Cystidean, on. 

 the shell of a Brachiopod. After Lutkeu- 



Fig. 133. Pseudocri- 

 nus, a Cystidean. 

 After Liitken. 



body being angulo-spherical, composed of solid plates. The 

 Cystideans (Figs. 132 to 134) originated in the Cambrian for- 

 mation, attained their maximum development in a number 

 of species in the Silurian, and became mostly extinct in the 

 Carboniferous period. They are the primitive Echinoderms. 



CLASS I. CRINOIDEA. 



Spherical or cup-shaped Echinoderms, without a madreporic plate, 

 ally attached by a jointed stem, a few free in adult life, with five arms sub- 

 dividing into pinnules; the ambulacral feet in the form of tentacles 

 arising around the mouth in, the furrows of the calyx or situated on the 

 jointed arms. In the Blastoidea and certain Cystideans the arms are ab- 

 sent, but the pi nnulce are usually present, though absent in Caryocystites. 

 Circulatory, water-vascular, and sexual organs much as in other Echino- 

 derms ; the digestive canal ending in a distinct eccentric aperture. 



