254 C. K Beecher —Origin and Significance of Spines. 



II. As extreme results of progressive differentiation of previous 

 structures. (A,,, B 3 .) 



The differentiation of existing ornamental structures into 

 spines has already been noticed in several instances in this 

 article. It was shown that spines often arise by the elongation 

 of nodes and tubercles or similar structures, by rhythmic alter- 

 nating areas of growth in lamellae and ridges, and by the 

 growth of matter at the intersections of lines, lamellae, ridges, 

 etc. Furthermore, it was indicated that this progressive differ- 

 entiation could be produced either (a) by the direct action of 

 external stimuli affecting the amount of nutrition brought to a 

 certain structure, (b) by the stimulus and dispersion of growth 

 force, or (c) by a combination of the two forces. In this dif- 

 ferentiation of the features which are generally called "orna- 

 mental," it will also be shown that the spine is the final result 

 of progressive differentiation and, as previously indicated, can 

 be formed out of a variety of other structures. The term 

 "ornamental" is mainly one of human interpretation, and is 

 used simply in apposition to "plain" or "simple"; for 

 example, a clam cannot be imagined as consciously favoring a 

 particular kind or arrangement of tubercles for ornamental 

 purposes. 



In a reticulate or cancellate surface formed by the crossing 

 of raised lines, ridges, or lamellae, it is evident that the causes 

 or forces producing such structures will be increased at the 

 points of intersection, and normally the amount of growth will 

 here be greatest. In this way, it is possible to account for the 

 very common presence of spines at the intersections of the 

 radiating and concentric lines on many Mollusca and other 

 organisms. 



A few examples will now be given illustrating the differen- 

 tiation of various structures into spines. 



The points of intersections of the elements of the lattice in 

 the Radiolaria are where spines are most frequently found ; as 

 in Lamacalpes lentellipsis, Orosphcera Huxley i, Carposphwra 

 melitomma, etc. 26 In Xiphosphwra pallas, the ridges about the 

 openings or meshes are granular, and the intersections are 

 raised into spines. Many of the discoid shells have their edges 

 differentiated into spines, as Heliodiscus asteriscus, H. cingil- 

 lum, II. glyphodon, Sethastylns dentatus, Heliodrymus den- 

 drocyclus, etc. When an edge becomes elevated and defined 

 as a carina, this structure is also often spiniferous, as in Tripo- 

 calpis triserrata and Astropilium elegans. The final differen- 

 tiation of the radiate arrangement in the Radiolaria results in 

 forms consisting only of a composite spine, 'as in the legion 

 Acantharia. 



In the Foraminifera, there are many instances of the gradual 

 differentiation of carinse, ribs, costee, etc., into spines. In 



