THE SENSORY RESPONSES OF CHITON 167 



affect only the very largest chitons. Barnacles remain attached 

 to a valve in some cases until they have formed three growth 

 lines ('year lines'). One effect of the barnacles is important in 

 connection with our preceding remarks regarding the forward 

 growth of the tegmentum as correlated with the erosion of the 

 superimposed umbo. Instances such as that illustrated in figure 

 4 show how it is possible for the shell to grow in a protecting 

 manner. In studying shell variation in the chitons it must be 

 remembered that the attached barnacles may be removed, before 

 or after their death, and leave no obvious trace, although they 

 may have been responsible for irregular growth of a valve. 



At sexual maturity the female Chiton tuberculatus is colored 

 in a different way from the male : its tissues are impregnated with 

 a salmon-pink substance concerned in the metabolism of the 

 ovary. If the shell plates are separated, this differential color- 

 ation of the sexes may be detected in dorsal view. Normally, it 

 is quite invisible. This is the first instance of its kind which 

 seems to have been described among mollusks. Its importance 

 has been discussed in another place (Crozier, '19). 



In the gill channels and under the girdle of chitons obtained 

 on sunlit shores where Enteromorpha and associated plants are 

 growing in a felted covering over the rock, there are nearly always 

 to be found considerable numbers of a commensal isopod. It 

 appears to be the Eusphaeroma (Sphaeroma) crenulatum of 

 Richardson ('02, p. 292; '05), described by her from specimens 

 collected at Bermuda many years before by Goode, but concern- 

 ing which no information as to habitat or local manner of occur- 

 rence has previously been recorded. The association of this 

 isopod with C. tuberculatus is general throughout the Bermuda 

 area, but the commensalism is of a more or less facultative kind, 

 since the isopod is found sometimes among the algae at some 

 distance from a chiton. Even where the supply of algae is 

 scanty (as in crevices within the walls of caves), the isopods are 

 also sometimes found, but usually not in such abundance, under 

 the girdle of Chiton. As many as twenty or more are to be found 

 under a chiton 8 cm. long. The association is quite independent 

 of the sex and sexual coloration of the chiton. The isopods are 



