INTRODUCTION, Xlll 



two layers of fibre that compose the column-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- 

 THID^e, Turbinolia, &c), the body tapers gradually to a 

 point below, without 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. crassicornis 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 {bell is, 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 warts, 

 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, 



