Till: ST KM VXD V.UA X. 



1 ; 



Platycrinus, though in both cases the base is monocyclic, tripartite, and symme- 

 trical, as was pointed out by Beyrich 1 . The anteroposterior axis which passes 

 through the anus and the opposite or anterior ray is called by him the radial axis, 

 while that which divides the base symmetrically is known as the dorsal axis. The 

 accompanying diagrams show that, while the dorsal axis of Platycrinus passes along 

 the left postero-lateral ray, that of the Blastoid passes along the right postero-lateral 

 ray (Fig. I.). It remains to be seen how far this rule holds good in the two remarkably 

 asymmetrical types Eleutkerocrinus and Astrocrirms, which will be best considered 

 later. But so far as the regular Blastoids are concerned, we know of no genus 

 which does not conform to it, though individual exceptions may occur, as will be 

 explained subsequently. 



It will probably conduce to clearness and brevity if we follow the plan which was 

 first introduced by Professor Lovcn, and has since been very generally adopted by 

 Echinoderm students, and employ a special designation for each of the rays in the 



U IIII.1 



Diagram showing the arrangement of the basals : A, in Platycrinus ; 13, in the Blastoids. 

 A, B, C, D, E. The five radii of the calyx, x. The small azygos basal, y, z. The two larger 

 basals. d . . . d. The dorsal axis, r . . . r. The radial axis, in which the anus is 

 situated. 

 The arrows show the (probable) direction of the spirally coiled digestive tube. 



organization of a Blastoid. It has been proposed by one of us 2 to denote each of 

 the five ambulacra of a Crinoid by the letters A to E. When an endocyclic Crir.oid, 

 such as Pentacrinus, is placed with its ventral surface upwards and the anal inter- 

 radius nearest to the observer, the opposite ray may be called the anterior one. Let 



1 " Ueber die Basis der C'rinoidca brachiata." Monatsber. k. preuss. Akad. Wiss. Berlin, 1871, pp. 42, 51. 



2 ' On the genus Actinometra, Mull., with a Morphological Account of a New Species from the Philippine 



Islands.' Trans. Linn. Soc, Zool. 1879, vol. ii. p. 26. 



