276 



PHYLUM ECHINODERMATA. 



In Marsupites there is a central plate in the middle of the five infra- 

 basals (Fig. 191). There is a central plate also in the larva of Antedon, 

 which however lie at the attached end of the stalk : it is called the 

 dorso-eentral and was once supposed to be comparable with the central 

 plate of other Echinoderms. It is not certain whether the central plate 

 in the calyx of Marsupites is this plate or the top segment of a larval 

 stem. In Antedon the central plate of the adult is a composite structure 

 consisting of the top piece of the stem and the infrabasals : it is called 

 the centro-dorsal (Fig. 189 Cd.). 



The number of radials in the apical system is normally five, but 

 the basals and infrabasals may be reduced to four, three or two. 

 The reduction of the basals is supposed 

 to be specially characteristic of the 

 ancient Crinoids ; all living Crinoids 

 have five, except Hyocrinus which has 

 three.* In living Comatulidae except 

 Atelocrinus neither the radials nor the 

 basals appear on the surface ; the basals 

 are united into a small plate, the rosette 

 plate, which encloses the chambered organ. 

 It frequently happens that the regular 

 radial symmetry of these simple calyces is 

 disturbed by the enlargement of the 

 posterior basal and by the presence of an 

 extra plate or of extra plates between the 

 radials which border the posterior (anal) 

 interradius (Fig. 190, ian). The anal inter- 

 radial plates have been much discussed. 

 When there is only one it rests upon the posterior basal and 

 lies between the right and left posterior radials ; it is called the 

 special anal (Fig. 190, ian). When there is more than one, the 

 lowest of them that which intervenes between the radials and 

 reaches the circle of basals is called the radianal (Fig. 201, ra), 

 the rest being simply plates of the anal tube, of which the 

 lowest is sometimes called the special anal plate. The irregularity 

 of the calyx caused by this peculiarity of the anal interradius is 

 never found in living forms f or in their allies, and was supposed 

 to be generally characteristic of palaeozoic Crinoids. There are 

 however, many of these in which it is not found. 



FIG. 192. Poteriwrinus 

 (after Zittel) showing a 

 simple calyx with dicyclic 

 base. There are two primi- 

 brachs but neither enter 

 into the composition of 

 the cup. 



* Fused to one in Gephyrocrinus, the ally of Hyocrinus. 



t For the asymmetry of the living Thaumatocrinus see p. 301. 



