THE CRINOIDEA 



107 



distal end of the stem after two main plans: (1) Deposition of 



solid, unjointed stereom, around the distal columnals, forming an 



encrusting plate or mass (Fig. CXIII. 2) ; this 



occurs on rocky bottoms. (2) Outgrowth of jointed 



branches from the plated end, forming " radical 



cirri," often with traces of polymeres like those of 



the primitive stem, often very long and branching 



again, and always with a lumen which contains 



an extension of the axial cord (Fig. CXVI.) ; this 



is adapted to a muddy bottom. The radical cirri 



arise from the vertical suture-lines of the stem, by 



the intercalation and outgrowth of small plates, 



and the extrusion of the axial cord (Fig. XVI. 3). fifa, after wyviiie- 



T ,v r i , ,, . . j n Thomson, x 20 diam. 



In the course ot race-history the cirri gradually ap- C oi, fascicuiar stereom 



FIG. XV. 



Distal end of stem 

 of larval Antedon bi- 



pear higher and higher up the stem (Fig. XVI. 1), 



and at the same time become shorter, more mobile, 



and eventually arranged in whorls (Fig. XVI. 5). At these levels 



the axial cords of the stem swell out, forming a repetition of 



the chambered organ (Fig. XVI. 4). 



The columnals are rarely all of the same height ; certain con- 

 spicuously larger ones, including those that bear cirri, are termed 



Fir,. XVI. 



Evolution of cirri. 1, part of stem of .1 Silurian crinoid with large, branching cirri (Brit. 

 Mus. E1354). 2, section across stem of a Carboniferous crinoid, showing branch from axial 

 canal to cirrus (Brit. Mus. E6708). 3, root of Jlarycrinus, with cirri originating between 

 pentameres (modified from Wachsmuth & Springer). 4, section across stem of /MCfiHtt* 

 WyvUle-ThmnMni at, lovel of cirrus-whorl, the central portions disproportionately enlarged 

 for greater clearness. 5, part of stem of Isocrinus decants, with cirri in whorls of live. 



"nodals"; those between them " internodals " (Fig. XVII. 1). 

 Nodals are the first columnals to be formed during growth ; 

 internodals are subsequently intercalated between them, and 

 again fresh internodals between the first formed internodals, and 

 so on. Fresh nodals are developed at the proximal end of the 

 stem, so that in that region are more nodals, while distally are 

 more internodals (Fig. XVII. 2 and 3). In one type nodals are 

 introduced immediately beneath the base of the cup, so that the 



