CYSTOIDEA. 129 



The division of the Crinoidea into stalked and free forms is 

 in many respects inapplicable as a basis of zoological classifi- 

 cation. There can, however, be no doubt but that the free 

 Crinoids are structurally an advance upon the fixed forms. It 

 is, therefore, of interest to note that the stalked Crinoids had 

 attained their maximum in the Palaeozoic Rocks, and had even 

 commenced to decline before the free Crinoids first made their 

 appearance in the Mesozoic Series. 



ORDER V. CYSTOIDEA. 



The Cystoidea are Echinodermata in which the body was 

 enclosed in a series of calcareous plates accurately articulated by 

 fyeir edges, and was fixed to the sea-bottom by. a longer or shorter 

 flexible jointed stalk. There were rarely true arms, and if present, 

 these structures are much less developed than in the true Crinoids. 



Fig. 82. Hemicosmites pyriformis, one of the Cystideans. The right-hand figure 

 shows the upper surface of the calyx. 



In general form the Cystideans are globular, oval, pear- 

 shaped, conical, or sub-cylindrical, and they resemble the 

 Crinoids in consisting of a stem or " column " and a body or 

 "calyx." The column is composed of a succession of cal- 

 careous joints, and in no respect differs from the column of 

 the Crinoids. In Lepadocrinus (fig. 83, D), however, it is 

 doubtful if the column was affixed to any foreign body, for its 

 lower extremity is composed of a single, long, spindle-shaped 

 piece. The stalk also is asserted to be absent altogether in 

 Sph&ronites pomum and in the genus Agelacrinites. The calyx 

 consists of a number of polygonal calcareous plates accurately 

 fitted together, and enclosing all the viscera of the animal. 

 Sometimes the plates are indefinite in number and arrange- 

 ment ; but in other cases the number of plates is limited, and 

 they are arranged according to a definite plan. In other cases 



