20 



ridges and furrows which form new points of articulation or insertion in pro- 

 portion as the area of the joint itself expands, always insured an equal firmness 

 of adhesion, for had the number of radii contained within the tirst narrow circle 

 continued without dividing as they passed through those successively formed 

 beyond it, being thinly* scattered over a surface so much wider, they would 

 scarcely have been adequate to this purpose. 



The nearly flat joints in the enlarged part of the column (PL. iv. fig. 7. 

 to 11) have on each side, near the centre, a concave depression, through which 

 the alimentary canal passes. To this a sphincter like muscle (Pi* iv. fig. 2L) 

 was probably affixed, sending off short fibres to the joint above, to aid longti- 

 tudinal action, and also radiating laterally, so as to spread in a thin membrane 

 over the space between the columnar joints attaching itself to their radii, and 

 more firmly adhering to the circular rings where these divide, and ultimately 

 perhaps connecting itself with the muscular membrane that covers the joints ex- 

 ternally. The sphincter seems to have been incapable of close contraction, 

 whereby it would have interfered with the free passage of the alimentary 

 canal, but its partial action on the radiating portion pulled downwards, or rather 

 more closely together the joints on one side, whilst its passive relaxation suf- 

 fered the other to rise, thus bending the column in various directions. The par- 

 tial contraction of the sphincter muscle between each two joints, together with 

 the longitudinal fibres, transmitted from one sphincter to the next, and thus pro- 

 ceeding through the interior of the whole column, formed a series of imperfect 

 balls or cones, distantly analogous to those gelatinous ones between the verte- 

 tebrac of fish, and not improbably here also tending to aid motion. Minute vessels 

 appear to have carried nourishment from the alimentary canal to the perios- 

 teum, or the investing membrane, which, by secreting calcareous matter, formed 

 the various joints. The growth or increase of these was effected by the con- 

 tinual application of fresh lamina?, as is demonstrated by the rings noticed 

 above intersecting the radiating stria, which may be also traced through the 

 interior by lines of a different colour, on forming transverse or longitudinal 

 sections. (PL. m. fig. 3.) They are fewer in small specimens, and more nu- 

 merous in large ones, resembling thus the rings which distinguish the growth 

 of trees. That the power of increase was nearly the same in all the animals of 

 this species of Apiocrinites, may be proved from a section showing 3 or 4 rings, 

 since these will occupy exactly the same space in the centre of one showing 



