764 Comparative Animal Physiology 



portantly to the mechanical adaptation of these organs to changes in light 

 intensity. Only among the crustaceans, however, has clear evidence been 

 presented that hormones are involved in the control of these movements (see 

 Chapter 11, p. 388). 



The compound eye of crustaceans is made up of a number of units, the 

 ommatidia (Fig. 289). Each ommatidium possesses three functionally dis- 

 tinct groups of pigments. The distal retinal pigment, melanin, is located in 

 cells which surround the distally placed dioptric apparatus of the omma- 

 tidium. In the dark-adapted eye this pigment occupies only a distal posi- 

 tion; the pigment disperses proximally in daylight to envelope the whole of 

 the ommatidium as far as the retinula elements. The proximal pigment, 

 which is also melanin, is located in the retinula cells and migrates to a po- 

 sition proximal to the basement membrane in the dark, at night, and distally 

 to meet the distal retinal pigment in daylight. In the light-adapted state the 

 whole ommatidium is therefore enclosed in a light-absorbing sleeve of pig- 

 ment. A third pigment, the reflecting white, appears to be particulate guanin. 

 In darkness this pigment occupies a position surrounding the retinula ele- 

 ments, thus constituting a functional tapetum. It migrates to a position proxi- 

 mal to the basement membrane in daylight. 



Investigations to determine the extent to which the pigment cells of the 

 right and left eyes of an individual are capable of independent responses to 

 illumination have led to various results. The more recent experiments of 

 this type have led to the conclusion that there is at least a partial inter- 

 dependence between the two eyes, a darkened eye becoming more or less 

 light-adapted when the contralateral eye is subjected to illumination.^^ Num- 

 erous observations have also indicated that one or more of the retinal pig- 

 ments of numerous species of crustaceans may undergo diurnally rhythmic 

 alterations in" their position in animals kept in constant conditions in respect 

 to illumination, especially in constant darkness.-^- ^^^ Leander kept on an 

 illuminated black background has been observed to show a dorso-ventral dif- 

 ferentiation in position of retinal pigment, apparently the result of the con- 

 siderably lesser illumination of the ventral than of the dorsal elements of the 

 eyes.^^ These various responses of the retinal pigments appear to suggest 

 that the control of the retinal pigments is not a simple one, but probably in- 

 volves a direct reaction of the retinal pigment cells to illumination and, in 

 addition, endocrine and possibly also nervous activities. 



There appears to be good evidence that the sinus gland of the eyestalk 

 produces a hormone which influences the position of the retinal pigment.*^ 

 163, 164 YJ^is J^as been called retinal pigment hormone or RPH.^*^ Injection of 

 extracts of the eyestalks of light-adapted Palaemonetes into dark-adapted 

 specimens kept in darkness induces in the latter a movement of the distal and 

 reflecting pigments to the position characteristic of the light-adapted state. 

 The eyestalks of all crustaceans which have been examined show the pres- 

 ence of RPH in larger or smaller quantities. That this hormonal principle 

 is normally concerned with the retinal pigment movements is indicated by 

 the fact that extracts of eyestalks taken from light-adapted individuals show 

 a much greater RPH content than do eyestalks taken from dark-adapted 

 ones.^^ However, it has been reported that the diurnal variation in retinal 

 pigments of certain grapsoid crabs persists even following removal of the 



