THE BUILDING OF THE DERBYSHIRE LIMESTONE. 



Crinoids belong to the large, and, to the geologist, important 

 group of animals termed Echinodermata. The soft and living 

 portion of the animals possesses the power of separating calcium 

 carbonate from the sea water, and forming from it a skeleton 

 made up of a large number of separate parts. A typical Crinoid 

 consists of a crown, stem, and root. In some forms, the stem is 

 absent, the crown swimming freely. The crown consists of a calyx 

 and dorsal cup, the former containing the viscera and arms, the 

 latter being made up of two circlets of five calcareous plates. 

 The whole organism exhibits a five-fold symmetry, five radial 

 planes passing through it longitudinally. 



The crown surmounts the stem, which, like the arms, is com- 

 posed of numerous ossicles or joints — circular, pentagonal, or 

 elliptical in form (Fig. 4) — and perforated by a circular or 



Fji;. .(. Stem Ossici.es of Crixoids. 

 {a. /'.) Genera uncertain (Carboniferous, Eng.) 

 (c.) Pentacrinus (Lias, Eng.) 

 {J) Platycrinus (Carboniferciis, Eng.) 



pentagonal axial canal. They are connected by a fibrous organic 

 tissue. The stem grows in length by the intercalation at the end 

 nearest the crown of new ossicles which increase in size. It is 

 surrounded at intervals by whorls of cirri, having the same struc- 

 ture as the stem, and the mode of attachment to the sea-bottom 

 differs in different genera according to the nature of the bottom. 

 Some stems terminate in a flattened expansion adapted to a rocky 

 bottom. Others terminate in a number of radicular cirri, adapted 

 for an oozy bottom. 



The mouth is at the centre of a membrane on the ventral sur- 

 face of the calyx. The upper surfaces of the arms are provided 

 with food grooves lined with fine vibratile filaments called cilia. 



