ORIGIN AND SIGNIFICANCE OF SPINES 



69 



the operation of some primary cause, and the derived or 

 secondary spines being produced, it is believed, by the law 

 of reiietition. 



The Radiolaria have already been frequently mentioned, 

 but as they are the most spiniferous of all classes of animals, 

 and represent the highest degree of spine differentiation 

 attained (figure 51 and Plate I), another brief notice will 



51 



52 



Figure 51. — Acontaspis bastala, a radiolarian; showing multiplication of 

 spines by repetition. X 200. (After Haeckel.) 



FiGLKE 52. — Strophalosia keokuk, an attached brachiopod; showing the 

 spines extending from the ventral valve to and along the surface of attachment. 

 X2. 



Figure 53. — A gastropod shell (Platyceras) to which are attached a number 

 of Strophalosia keokuk. Natural size. 



be of interest. These spines furnish characters of high 

 taxonomic value, although generally speaking they seldom 

 have more than specific importance among other classes. 

 The Echinoidea and Asteroidea must also be noticed in this 

 connection, though from the nature and origin of their spines 

 they do not conform to the mode of spine growth in other 

 classes. 



Productus, Productella, Strophalosia, Aulosteges, and Sipho- 



