viii ECHINODERMATA MORPHOLOGY OF SKELETON 375 



be muddy or sandy, the base of the stem puts out lateral branches, 

 the so-called root-cirri, the numerous ramifications of which penetrate 

 the sea floor in all directions. The end of the stem itself may at 

 the same time branch like the root-cirri. When the sea floor is 

 rocky, the root-cirri spread out more horizontally, accommodating 

 themselves to the surface to which they are attached, and becoming 

 cemented to it at their ends by means of a calcareous secretion. 



Further, it is almost certain that individuals of some species, e.g. Pentacrinidce 

 and some Palaeozoic crinoids, are capable of free locomotion when the stem is either 

 voluntarily or accidentally 

 broken. Such locomotion is 

 no doubt chiefly promoted by 

 the movements of the arms, 

 the cirri serving rather for 

 attachment. 



In Hoi opus (Fig. 250, p. 

 305) the stem is wanting. 

 The calyx, which resembles 

 a reversed cone in shape, is 

 cemented to the substratum 

 by means of an irregularly 

 expanded calcareous mass. 



The Antedonidtx are 

 stalked and attached 

 only in the young stage. 

 The larval stem re- 

 sembles that of the 

 Bourgueticrinidse (Fig. 

 326) ; the cirri are 

 developed only on its 

 uppermost ossicle, on 

 which five radially ar- 

 ranged cirri first appear, 

 then five interradially 



-la 



arranged. At a certain 



FIG. 326. Several stalked young stages of Antedon 

 phalangium (A) ; Antedon spec. (B) ; Antedon tuberosa (C) ; 

 and Antedon multispina (D), after P. H. Carpenter. For 



<!<', Points of attachment of the cirri. 



T~ . . , aim iiuuuuoii mm 



stage, differing in the lettering see p . 317. 

 different species, the 

 calyx together with the uppermost columnal (i.e. the one carrying cirri), 

 which is fused with the infrabasals to form the centrodorsal, separates 

 from the stem, the latter remaining attached to the place where it was 

 fixed. Above the cirri already formed, i.e. between them and the base 

 of the calyx, new whorls of cirri continuously appear on the centro- 

 dorsal, which constantly increases in size, so that we are tempted to 

 consider this piece as part of the stem, consisting entirely of nodal 

 ossicles fused together without intervening internodes. 



The Comatulidae can both swim by the rowing movements of their arms, and 

 creep by means of the cirri and of the arms. They can, further, anchor themselves 

 by their cirri, the arms being then directed upwards. 



