66 THE PINEAL ORGAN 



of " white pigment " and " black pigment " in well-defined and constant 

 positions in the cells composing the retinulae of the compound eyes of the 

 same animal indicates that the appearance of black pigment in Gammarus 

 ornatus is not merely due to the deposit in the retinal cells of concretions 

 of phosphate of lime, which on examination with the microscope by 

 transmitted light appear black, as has been said to be the case with the 

 white pigment in the pineal sense-organ of Petromyzon. In Gammarus 

 the white pigment of the " accessory cells " when viewed by transmitted 

 light appears as glistening refractile granules, not black. These 

 " granules," according to Parker, are not subject to the photochemical 

 changes which occur in the pigment cells which ensheath the colourless 

 rhabdome, and he suggests that " the accessory [white] pigment cells 

 probably act as reflecting organs and in very dim light turn such rays as 

 have escaped laterally from the rhabdome back again into that structure, 

 thus aiding in an effective stimulation of this organ." Parker's description 

 of these two kinds of pigment contained in separate cells lying adjacent 

 to one another, one subject to physico-chemical changes produced by 

 exposure to light and the other merely serving as a reflecting mechanism, 

 suggest that the " black " granules in the dark cells are true pigment 

 granules containing melanin, while the granules of the " white " accessory 

 cells are possibly due to a fine crystalline or crystalloid deposit in the cell 

 substance. 



The variability in the colour of eyes is a subject of great interest and 

 importance. The eyes of many invertebrates are red. Thus in the 

 Protozoan Euglena viridis, which is commonly found floating on the 

 water of ponds and gives to this a bright green colour, the eye-spot or 

 " stigma " is- a vivid red, in contrast to the general green colour of the 

 body. The red colour is said to be due to a pigment allied to chlorophyll 

 and called haematochrome. In some orders of the coelenterates, the ocelli 

 or eye-spots, arranged round the umbrella, appear as brilliant dots of 

 colour, orange or red, sometimes phosphorescent. 



Among the Annelida, or ringed worms, Andrews ' has described in 

 Tubicola potamilla highly differentiated compound eyes of a bright orange- 

 red colour on the sides of the branchiae. This animal lives in a leathery 

 tube which is seen projecting from holes in gasteropod and bivalve shells. 

 From the end of this tube cephalic, branchial plumes expand as a circular 

 series of radiating stems, each bearing two rows of branchial filaments 

 which are in the fully expanded state, directed forward. The eyes are 

 on the posterior or outer sides of the main stems, there being rows of 

 three to eight on each of the 20 stems. Each eye is a convex hemispherical 

 protuberance on the outer side of the main stem. On section the eyes are 

 1 E. A. Andrews, J. Morph., 5, p. 271, 1891. 



