INTRODUCTION. 
Xlll 
two layers of fibre that compose the columu-wall, — the 
outer transverse fibres becoming concentrically circular; 
the inner longitudinal ones converging to, or towards, a 
centre. In general, the boundaries of these divisions are 
distinctly marked by an abrupt angular change of the 
direction of the inner fibres ; but in some species (Ilyan- 
THIDJE, Turlinolia, &c.), the body tapers gradually to a 
point below, wdthout any angular change of direction. 
The fibres of the inner layer meet at a central point in the 
base, except in those species which have a central foramen 
there ; but in the disk they sustain another change of direc- 
tion, bending abruptly down at right angles, so as to form 
an inclosure in the axis of the column, parallel to the outer 
wall — the fibres of the outer layer still coating them. This 
downward prolongation forms the stomach, which will be 
presently described. 
In T. crassicoi'Tiis the angle which is formed by the in- 
bending of the fibres to form the disk, is strengthened by a 
muscular cord, about half a line in thickness, consisting of 
annular fibres, and evidently acting as a sphincter : it is this 
band that forms the parapet. 
In Sagartia {bellis, miniata, nivea, &c.) the muscular 
tunic, in contraction, corrugates into a reticulate or honey- 
comb-like pattern, inclosing shallow cells of much regu- 
larity. It is, I think, these inclosed areas, any one of 
which may be considered as a cell, with perpendicular 
walls of muscular tissue, that constitute the sucking w'arts, 
by means of which minute fragments of shell or gravel 
are grasped, and retained with considerable force. If this 
exposition is correct, all of the corrugated cells are capable 
of becoming suckers at the will of the animal ; but, in fact, 
only a few are so used at a time. The cells {nivea, miniata) 
are about '014 inch in depth and longitudinal diameter, 
while their transverse diameter may average about *084 
inch. It is the outer layer of muscles that constitutes 
these corrugations. 
The sucking warts in the Bunodidce, are of similar 
character ; but here the elevation of the muscular tunic is 
more permanent, and the walls of the individual cells are 
thicker, and are incurved towards each other. 
To the muscular system belong the Septa. These are 
thin plates of muscular tissue, comprising the two layers of 
transverse and longitudinal fibres, doubled on each other. 
