NOTES ON SOME PROBLEMS OF ADAPTATION. 



251 



TABLE II. 



RELATION OF ASYMMETERY (EXCESS NUMBER OF GILLS ON ONE SIDE) TO SIZE 

 (CLASSES ACCORDING TO LENGTH OF VALVE 4.) 



Class Centers. 



There is no apparent correlation of asymmetry with sex or with 

 habitat. In the majority of cases the number of gill-plumes differs 

 on the two sides, but the difference is usually not more than one. 

 The asymmetry can therefore hardly be connected with the torsion 

 of the body in gasteropods. 2 



Summary. In Chiton tuberculatus at Bermuda the number of 

 ctenidia increases from 32 on either side, in individuals about i.o 

 cm. long, to an average of 49 on a side in individuals of the largest 

 size (910 cm. long). The increase in the number of gills is at 

 first quite rapid, so that the maximal number is achieved before 

 the animal is 6.0 cm. long. The mean number of ctenidia is a 

 little less in females than in males of the same size or age. 



On an exposed ocean beach these chitons have the same number 

 of gills as found in the population of an inclosed sound. 



Minor asymmetries in number (i to 5) are the rule when the 

 two gill series of an individual are compared. Asymmetry follows 

 a random distribution; it is an accident of growth and has no 

 morphological significance. 



CITATIONS. 



Arey, L. B., and Crozier, W. J. 



'19 The Sensory Responses of Chiton. Jour. Exp. Zool., Vol. 29, pp. 

 157-260. 



~ Two instances were found of a curious malformation, which may be 

 noted here. At the level of the fourth valve a tough " bridge " of tissue had 

 grown across from the lateral wall of the foot to the girdle, forming an 

 arch over the ctenidial channel. These " bridges " were about 3 mm. wide. 



