CRINOIDEA. 571 



especially Palaeozoic, the adjoining edges of the first radial plates are 

 partially separated by the circle of first interradials, and in the living 

 Tkaumatocrinus, and the extinct Palaeozoic family Rhodocrinidae -, an 

 interradial alternates with a radial and touches the basals. Higher orders 

 of interradials are found in many Palaeozoic and some Mesozoic species, 

 but in the latter, as well as the living forms, the interradials, when present, 

 are as a rule numerous and irregular. 



The oral system is represented by a circle of five orals surrounding 

 the mouth in Thaumatocrinus, Holopus, Hyocrinus and Rhizocrinus. Present 

 in the pentacrinoid, i. e. stalked Antedon, they are resorbed during growth. 

 They are large, and their circle is broken by the anal tube in many 

 Palaeozoic genera, in many of which an oro-central is present, and to- 

 gether with the circle of orals forms a vault ', dome, or tegmen calycis arching 

 over the mouth. The extinct Actinocrinus has in addition circles of dome 

 radials and interradials, but as a rule such plates are small and irregular. 

 In all these forms the ambulacral grooves ascend the dome, and enter the 

 oral vestibule between the bases of the orals. 



The surface of the disc between the ambulacra and their divisions, 

 as well as its continuation upon the arms, is sometimes naked, though 

 calcareous spicules are present at the edges of the ambulacra. It is, 

 however, as a rule protected by irregular anambulacral plates, continuous 

 at the edges of the disc with the interradials. The edges of the ambulacra 

 themselves are generally guarded on the disc, arms and pinnules by a 

 series of covering plates (adatnbulacrals) which in some Comatididae are 

 represented by soft lobes. Whether plates or lobes, they alternate inter se 

 on opposite sides of the grooves over which they can close. They are 

 often borne upon the edges of a single series of side plates. 



The plates of the calyx, disc and ambulacra develope as cribriform 

 calcareous films, and are thickened by addition of new films. 



To the radials are attached the arms, and to certain joints of the 

 stem, the cirri. The arms generally branch dichotomously, and the joint 

 known as axillary, which bears the branches, has two equal oblique and 

 terminal facets, one for each branch. The growing point of the arm or 

 its branches forks at short intervals, and one division of the fork, 

 alternately the right and left, remains relatively short and small con- 

 stituting a pinnule 1 . In the living Hyocrinus, however, and some fossil 

 forms, the pinnule is about as long as the part of the arm distal to its 

 point of origin. Such pinnules resemble the branches of an arm, but the 



1 Pinnules are absent from every axillary joint, and from the proximal of two joints united by 

 ligament or by syzygy. The lowest brachials of Atelecrinus (Comatulidae] and of Bathycrinus and 

 Rhizocrinus are also devoid of them, and in Antedon it has been observed that the pinnules of the 

 lowest brachials (3-7) are developed at a late period. Hence it appears that pinnules may be formed 

 independently of the growth of the tip of the arm. 

 t Oo 



