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 first 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. tv. 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 (Pu. rv. fig. 21.) 
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 inteérior of the whole column, formed a series of imperfect 
balls or cones, distantly analogous to those gelatinous ones between the verte- 
tebrz 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 lamine, 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. (Pu. 111. 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 
