56 STUDIES IN EVOLUTION 



spiniferous. Others, like Lithodes maia, have the spines 

 generally distributed over the carapace and legs. While 

 serving for defensive purposes, this generally spinose char- 

 acter has probably reached its extreme develojDraent through 

 the influence of repetition (B„). The nauplius larva of 

 Lepas fascicularis is very large, and has highly defensive 

 spines which are explained by Balfour ^ as a secondary ad- 

 aptation for protection. The larger spines on Trilobita, 

 especially those from the genal angles and the axis, doubtless 

 served protective purposes. The extremes of spinosity in 



44 



Figure 44. — Zoea of the common crab {Cancer irroratus) ; lateral view. 

 X8. (After Verrill and Smithes.) 



this class are found in the various species and genera of 

 the family Acidaspidse, and also in many forms of Arges, 

 Terataspis, Hoplolichas, etc. 



Even among the star-fishes, which are so generally spinose, 

 some forms have the spines so prominently developed on the 

 most exposed portions of the animal that they evidently 

 serve for protection ; as Acanthaster Solaris, Echinaster 

 spinosus, etc. 



The examples already given are sufficient to emphasize the 

 fact that after spines are developed they may then often 

 serve for protection and offence, and therefore be useful, their 

 efficiency being controlled by natural selection resulting in 

 the survival of the fittest. 



