C. E. Beecher — Origin and Significance of Spines. 15 



cease, and the surface of the organism would inversely revert 

 to an early and more primitive type without spines. Normally, 

 these changes would represent the infantile, adolescent, mature, 

 and early and late senile periods of the life of the organism. 

 In some cases, however, the stages of spine growth, or acan- 

 thogeny, do not agree with the ontogeny of the entire individual 

 in respect to time, and here acceleration and the phytogeny of 

 the species will be found to offer the proper explanation of the 

 divergence. 



As simple examples of the ontogeny of spiniferous species, 

 the Mollnsca afford especial advantages, owing to the fact 

 already noticed, that the stages of development are commonly 

 preserved in a single individual. In figure 2i, the larval shell, 

 or prodissoconch, of Pelecypods, or bivalve shells, is repre- 

 sented, and shows the usual type throughout a large portion of 

 the class. The succeeding shell growth of the dissoconch is at 

 first generally smooth, save for the fine concentric lines of 

 growth (figure 25). In ornamented or spinose species, how- 

 ever, irregularities in the growth lines soon appear (figures 26, 

 27"), and these shortly assume the characteristic surface sculp- 

 ture of the normal adult. Thus, the prodissoconch of Avicula 

 sterna is represented at p, figure 25, and is followed by regular 

 concentric growth during the nepionic stages. In figure 26, 



24. 



25. 



26. 



Figure 24. — Prodissoconch of Ostrea virginiaaa. x 43. 



Figure 25. — Each stage of Avicula sterna ; p, prodissoconch. x 19. 



Figure 26. — Yonng Avicula sterna, showing the beginning of spine growth. 

 x3. 



Figure 27. — Young Saxicava arclica. x 19. 



Figure 28. — Young Anoima aculeata ; p, prodissoconch succeeded by early 

 smooth and later spinous dissoconch growth, x 30. (24-28 after Jackson.) 



