G. E. Beecher — Origin and Significance of Spines. 7 



Often there are various kinds of surface ornaments, which 

 by growth and differentiation develop into spines. By rhyth- 

 mic, alternating areas of accelerated or retarded growth, the 



Figures 1-5. — Different stages in the growth of a spine. 1, plane surface ; 2, 

 slight elevation ; 3, node ; 4, short spine ; 5, completed simple spine. 



concentric laminse on many molluscs may produce spines, as 

 shown in figure 26. In the same way, the radiating ridges may 

 be diversified into a row of spines, as represented in figure 6. 

 Further, the surface may be reticulate, with longi- 

 tudinal and transverse lines, and at the points of 

 intersection, nodes and often spines are formed 

 after the manner shown in figures 7-12. The longi- 

 tudinal or vertical lines may become obsolete, 

 leaving the spines to be borne on the transverse or 

 horizontal lines (figure 10). In other cases, the hori- 

 zontal lines disappear, leaving the spines on the 

 vertical lines (figure 11). Finally, both horizontal 

 and vertical lines become obsolete, and then only 

 the spines remain, as shown in figure 12. 



The indirect production of spines is not always 

 evident, for if the ontogeny or phylogeny of the 

 individual is unknown, its direct or indirect devel- 

 opment cannot be determined. An excellent 

 example of indirect, or regressive, growth of spines 

 is afforded in the common Barberry (Berberis vul- 

 garis), on the summer shoots of which are shown 

 Fig 6.— a most of the gradations "between the ordinary 

 profile of a sin- ] ea ves, with sharp bristly teeth, and leaves which 

 fid^o/Soon- are re duced to a branching spine or thorn. The 

 dylusprinceps, fact that the spines of the Barberry produce a leaf- 

 showing the bud in their axil also proves them to be leaves" 24 



tSspinef (%ure 13). 



It should be noted that the process of spine 

 development illustrated in Spondylus (figure 14) is directly 

 opposed to that of the Barberry. In the former, the initial 

 growth is smooth, then faint concentric and radiating lines 

 appear, which gradually grow stronger, developing more or 

 less regular inequalities ; and by the excessive growth of these 

 variations, spines are formed. In the Barberry, there are at 



