228 University of California PuhUcations in Zoology [Vol. 18 



plasticity may result in formation of new species. They further sug- 

 gest the possibility that different species occupying different geo- 

 graphical and bathymetrical areas are descendants from common 

 ancestral stocks and that their differences are the results of the par- 

 ticular external influences and of the degree of plasticity in each 

 species. Species with the greatest plasticity are best provided with 

 protective structures and are more widely distributed. Being able 

 to overcome vicissitudes and changes in environment, they are superior 

 to species specialized for certain modes of life and for certain environ- 

 ments. Lepidonotus squamatus serves as a good illustration. Tt has 

 rough spiny elytra (pi. 8, figs. 34-35) covering the entire dorsum, 

 and numerous strongly developed setae (pi. 7, figs. 8-11). On one 

 occasion Mcintosh (1900) observed that a specimen of Lepidonotus 

 squamatus which was in a tank with other animals was coiled up so 

 as to have its scales and setae placed to the best advantage for self 

 protection. When it was picked up by a young cod it was immediately 

 rejected and fell to the bottom. Again it was attacked by Cottus, and 

 again also immediately rejected, while Nereis and other annelids were 

 devoured by the same animals. This explains partly the world wide 

 distribution of Lepidonotus squamatus while the largest species of the 

 family, Lepidasth enia gigas, depending entirely on a commensalistic 

 life and being ill protected outside of the tube, has a very limited dis- 

 tribution. Probably young Lepidasth enia gigas fall victims to their 

 enemies if they do not happen to find a vacant tube of an Amphitrite ; 

 or their adaptability is so limited that they can live only in that 

 particular environment as commensals and perish in open ocean. 

 Hence there are no free-living forms of that species. These facts 

 show that, besides the external factors, the physical condition of the 

 animal, its powers of adaptation, and its variability are equally impor- 

 tant factors in its distribution. 



