vni ECHINODERMATA MORPHOLOGY OF SKELETON 373 



median zigzag line. These plates can be raised and depressed in the living animal ; 

 when they are raised the food groove is open, when depressed, it is shut. 



An altogether peculiar arrangement is found in the arms of the genus Crott<>- 

 crinus (Upper Silurian, England Sweden), which is thought by some to belong to 

 the Camerata. The free arms branch extraordinarily frequently, the separate branches 

 being closely crowded together, and forming together a wide expanded coherent 

 disc round the calyx, resembling the fully open corolla of a flower. As many as 500 

 to 600 branches may in some forms reach the edge of this disc (C. riigosus, Fig. 324). 

 Each ossicle of the arms has two lateral processes, which become connected with 

 similar processes of the corresponding ossicles of the neighbouring arms or branches, 

 so that the disc formed in this way by the skeleton of all the free arms is lattice-like. 

 At definite distances from the calyx the brachials are of equal length, so that they, 

 as well as the sutures which lie between the consecutive brachials, seem to be arranged 

 in regular concentric rings round the calyx. The whole brachial disc was very flexible, 

 and could be rolled up over the calyx from its periphery. In C. pulcher, the brachial 

 disc falls into five broad radial lobes, which, when the disc closes over the calyx, over- 

 lap like the petals of a bud. The food grooves are covered by double longitudinal 

 rows of alternating covering plates. 



(c) The Stem (Columna). 



The great majority of Crinoids are attached to the bottom of the 

 sea by means of a jointed stem. Among recent Crinoids only the Ante- 

 donidte and Thaumatocrinus are, in the adult condition, non-pedunculate 

 and unattached. The stalked condition is undoubtedly the more 

 primitive, for (1) the Crinoids show very markedly the habitus 

 characteristic of many attached animals, and (2) all free and unstalked 

 Antedonidce pass through an early stalked and attached stage. The 

 stem, which varies greatly in length and thickness, consists of a series 

 of calcareous ossicles one above the other, the uppermost of which is 

 connected with the centre of the apical system, and carries the calyx 

 with its arms. 



The ossicles of the stem (columnals) vary greatly in shape. They may be flat and 

 disc-like, or long and cylindrical ; sometimes they are gradually thickened towards 

 each end in such a way as to resemble dice-boxes. Further, the columnals in 

 different parts of one and the same stem may be very different. The external outline 

 of the ossicles in transverse section is sometimes pentagonal, sometimes round, 

 rarely elliptical. They are connected with one another more or less firmly by sutures, 

 or else are movably articulated. The stem throughout its whole length is pene- 

 trated by a central canal (axial canal), which thus runs through all the consecutive 

 columnals. Within this canal run the ccelomic canals (continuations of the chambered 

 organ) and nerves. The size of the canal in transverse section differs as much as 

 its shape. The outline of its section seems most frequently to be pentagonal or quinque- 

 lobate, but it is not infrequently round. Occasionally also the central canal is sur- 

 rounded by five narrower peripheral canals. 



New ossicles are added, as the animal grows, at the upper end ; at first they are 

 small and flat, and often concealed within the stem. The most constant place of 

 their appearance is between the uppermost columnal and the base of the calyx. 

 New ossicles may, however, also be intercalated between two already formed ossicles, 

 but this almost always takes place at the upper end of the stem. In a growing stem 

 the ossicles in the upper part vary greatly in length, the shortest being the youngest. 



