TYPE -ECHINODERMA. 



543 



stem cirri are entirely wanting except near the point of fixa- 

 tion. 



The uppermost plate of the stem is usually regarded as 

 forming the apex of the calyx and is termed the centrodorsal. 

 Above this comes in most recent forms a series of usually five 

 (sometimes three) iuterradial plates, the basals (Fig. 250, b\ but 

 in one genus, Thaumatocrinus, there occurs between the ceu- 

 trodorsal and basal plates a series 

 of five radial plates which are 

 termed the parabasals or under- 

 basals, and which have also been 

 found to occur in the embryo of 

 Antedon, later on fusing with the 

 centrodorsal. Succeeding the ba- 

 sals are from two to seven circles 

 of radials (r), each circle being 

 also composed of five plates termed 



the first, second, third, etc., radials FIG. 250. THE APICAL SYSTEM 

 according to their succession 

 counting from the centrodorsal. 

 In Antedon and some of its allies 

 the number of cycles of radials 

 seen from the exterior is one 

 short of the actual number which 



exists, the first radials being overlapped and covered in 

 by the second ; and furthermore in the same forms the 

 basals have also been pushed, as it were, within the calyx 

 and have fused to form a single plate, the so-called roxette 

 plate (Fig. 251, Ros), which rests upon the centrodorsal, par- 

 tially closing a cavity in that plate. The terminal radials 

 usually present two articulating facets in their distal sur- 

 faces and are generally known as the axillaries (Fig. 250, a), 

 since the arms articulate with them. In the genus Thauma- 

 tocrinus between each pair of first radials an mterradial plate 

 occurs, a condition frequently found in fossil genera (i), but 

 usually wanting in recent forms. These various plates which 

 constitute the apical system are united by sutures, the edges 

 of the various series of plates coming into contact, so that a 

 firm support is afforded for the arms. 



OP Melocrinus (FOSSIL) (from 

 BROWN). 



a = axillary plates. 

 b = basal plates. 

 i interradial plates. 

 r = radial plates. 



