590 RESPONSE TO SHADING OP CHROMODORIS 



oxygen no longer exists in the medium (McClendon 1917, p. 34); in 

 Cassiopea, according to Carey (1917), pulsations of the swimming 

 bell cease in sea water less alkaHne than pH = 7.9, but Chromodoris 

 moves about for a time in solutions more acid than this. 



The control of gill protrusion, and of the photic irritability of the 

 plumes by the existing alkalinity, which is prominent at the ex- 

 tremes of the nonnal range, is probably exercised, to a less con- 

 spicuous degree, under the normal variations of loss of COo from the 

 body of the nudibranch. The whole system of gill reactions is appar- 

 ently concerned fundamentally with regulating the loss of CO2 

 from the blood, analagous to the method whereby the respiratory 

 rhythm of a vertebrate is controlled, but with this difference: the 

 diffusion of CO2 from the respiratory surfaces is directly controlled in 

 Chromodoris by varying the amount of surface in contact with the 

 external medium. Upon this respiratory control of the gill move- 

 ments the property of branchial contraction as the result of shading is 

 superimposed; it is a method of response which is of secondary im- 

 portance. In order that the shading reaction may be exhibited in its 

 full amplitude, the gill-crown must be at a certain stage of exten- 

 sion, while the intrinsic reactivity of the individual plumes is also 

 limited by at least one of the factors which determine the degree of 

 gill-crown protrusion. The sensitivity of the plumes to a decrease in 

 light intensity is probably of some protective value. The gills are 

 bitten at by fishes, and in about 10 per cent of the individuals the 

 plumes are found to have endured some degree of injury, presumably 

 from this cause (cf. Crozier, 1919). Hence it is of interest that 

 the protective aspect of the branchial activity should clearly be a 

 secondary thing, in spite of the fact that injury is continually be- 

 ing suffered by the gills. It is entirely possible that under natural 

 conditions the photic irritability of the plumes should be a func- 

 tion of the reaction either of the sea water or of the blood of the 

 nudibranch. It has not been possible, however, to discover any 

 relation between the irritability of the gills in this respect and the 

 tint of the indicator pigment in the epithelium of the gills; in 

 different animals this tint is blue, purple, or even pinkish, pos- 

 sibly indicating variations in intracellular acidity (the indicator 



