of the Ciiiioidea brachiata. 399 



bas(;, and notliing of the relations of the base to tlie excen- 

 trical vertical aperture. 



In this way the first conception of the so-called " bilateral" 

 stvucture of the Crinoids originated (Cystidea, p. 2, note). 

 Subsquent authors retained the conception, but limited it to the 

 consideration of the deviations in the regidar arrangement of 

 the ladii, which certainly are connected with an excentric po- 

 sition of the vertical aperture (as F. Ivoemer in ' Lethaja Geo- 

 gnostica,' i. p. 221). iV S})ecial relation of the symmetrically 

 divided base to the symmetrical division of the radii was 

 ignored J nevertheless it exists, and is subject to definite 

 laws. 



The regular quinqu eradiate arrangement of the radii of 

 the calyx is converted into a symmetrical arrangement by 

 one of the five interradial directions becoming particularly 

 marked ; Avhen this occurs the radii arrange themselves sym- 

 metrically, in accordance with an axis which cuts through the 

 interradius characterized as polar and the opposite radius. In 

 many cases the distiftction of the polar interradius is perceptible 

 only by the vertical ajjerture taking up an excentric position, 

 by moving towards the side from the ventral pole in the direc- 

 tion of one of the interradii. In other cases, where the cover- 

 ing of the vertex between the radii which have grown together 

 to form the calyx is prolonged downwards, and forms the so- 

 called interradial spaces, that interradial space towards which 

 the excentric vertical aperture is directed acquires a composi- 

 tion different from that of the others, or pushes further down- 

 wards between the adjacent radii. But it also happens, as in 

 Actinocrini with the vertex prolonged like a proboscis, that 

 one of the five interradial spaces possesses the abnormal com- 

 position, whilst the vertical aperture has retained its central 

 position. As appears from this, it was incorrect to speak of 

 anterior and posterior in the Crinoids, as if the peculiar inter- 

 radius were always to be called posterior or anal, even when 

 the vertical aperture is central and when Ave have no know- 

 ledge of its position. Leaving out of consideration the ques- 

 tion Avhether the single vertical aperture is to be called the 

 mouthy after the example of Mliller and L. von Buch, or the 

 anusy as recent anatomists Avish, the anal interradius is still an 

 unsuitable term, because the distinction of the interradius is not 

 called forth by the vertical aperture, the excentric posi- 

 tion of the latter being rather only a consequence of the 

 fundamental condition of the polar distinction of one of the 

 interradial directions. Accordingly I shall call the axis 

 according to Avhich the radii arrange themselves symme- 

 trically the radial axis, in contradistinction to the dorsal 



