NOTES ON SOME PROBLEMS OF ADAPTATION: 9. 

 CTENIDIAL VARIATION IN CHITON. 1 



L. H. SNYDER AND W. J. CROZIER, 



(1901-) (1892-) 



ZOOLOGICAL LABORATORY, RUTGERS COLLEGE. 



I. The respiratory organs of the Amphineura comprise a more 

 or less extensive row of branchial filaments on either side of the 

 body, in the groove between girdle and foot. Within the species 

 it is known that the number of these ctenidia is variable. It is 

 likewise known that the right and left ctenidial sets of an individual 

 may differ in number of included gills. Published observations 

 on these points have been summarized by Pelseneer (1920, p. 201). 

 It appears that in some species the number of ctenidia may increase 

 with the size of the animal; in others it seems to decrease; while 

 in still others no correlation is very evident between size of animal 

 and number of gills. We have sought to determine the nature and 

 extent of ctenidial variation in Chiton tuberculatus of the Ber- 

 mudas, having in mind the possibility of evidence upon the follow- 

 ing points : 



(1) Ctenidial variation as correlated with size (age), 



(2) with sex, and 



(3) with habitat; and 



(4) indications of asymmetry. 



The gill plumes were counted in 100 individuals, of a range of 

 sizes. The length of the fourth shell-valve, along its mid-line, was 

 measured as an index of size. By means of graphs such as that 

 given by Arey and Crozier (1919, p. 164) the length of the fourth 

 valve may be translated into terms of the length of the individual ; 

 the curled condition of many of the specimens, as preserved, pre- 

 cluded direct determination of their lengths. It is sufficient to 

 note that the length of the shell-valve varies directly with the total 

 size of the animal, though the relation is not exactly a linear one, 



i Contributions from the Bermuda Biological Station for Research. No. 141. 



246 



