c UjAMOCRINI S DIOMEDiE. 65 



Fraas* has given ;i section of the stem of Millericrinus, showing (Plate 

 XXIX. Fig. 12) the modifications introduced into the arrangemenl of the 

 original limestone network b\ the longitudinal muscular threads. These 

 give rise on the laces of the joints of the Crinoid stem to ;i very varied 

 arrangemenl of the calcareous cells which have considerable systematic 



value. 



1 have on Plate XXVII. given views of the minute structure of the pin- 

 nules, of an arm joint, of the first radial, and of a pari of the stem. The 

 close agreement of the limestone network is apparenl al a glance. Figure 3 

 shows the structure of the meshwork, the closeness of the reticulation de- 

 pending mainly either upon the size of the meshes or of that of the con- 

 necting rods. In the pinnules and arm joints the reticulation is in curved 

 lines diverging from the axial canal of the pinnule or arm joints (Figs. 1, 2). 

 In the radials it is in straight lines, at right angles nearly to the outer 

 face, where the reticulation is coarser (Fig. 4). A longitudinal section 

 of the stem through one of the joints shows the vertical arrangement of 

 the limestone cells around the axis (Fig. 5), while a transverse section 

 (Fig. 6) shows the radiating arrangement of the reticulation from the 

 edge of the joint to the central canal. The darker spots of the lime- 

 stone network represent the cross sections of the rods at right angles to 

 the plane of the section connecting superposed cells. 



The Apical System of Echinoderms. 



As Neumayer t has pointed out, the difficulty of comparing the apical 

 system of Echinoderms with the monocyclic or dicyclic Crinoids introduces 

 at once difficulties which have given rise to many different opinions. 



In making the comparisons of the apical systems of Echini, Starfishes, 

 Ophiurans, and Crinoids, it has been customary to ignore the Holothurians, 

 and only the papers of Theel, of Semon, and of the Sarasins have given 

 us any grounds for not considering the Cystideans as the earliest stage 

 of Echinoderms. 



Starting from the assumption that the Crinoids represent this earliest 

 phase of Echinoderm development, it has been customary to homologize 



* Eberhard Fraas, Die Asterien d. Weissen Juras von Schwabeu u. Franken. Palaoutograph., 

 XXXII.. p. 237. 



f Morphologische Studien iiber fossile Echiuodermen. Sitzungsb. d. k. Akad. der Wiss., LXXXIV., 

 I. Abtheil, Juui Heft, 1881, p. 143. 



