288 



BULLETIN: MUSEUM OF COMPAKATIVE ZOOLOGY. 



tliey shorten only to about five sevenths of their original length in- 

 stead of to three sevenths. Thus their long retention in the dark 

 seems to have prevented a return to the more completely contracted 

 condition normal for the light. The fact that the more peripheral parts 

 of the cells in the contracted condition ai-e the more wrinkled indicates 

 that the axial part has retained its contractile nature more completely 

 than the peripheral parts, and suggests the idea that, since this axis 

 contracts in a definite direction, it must possess something of the nature 

 of a muscular core. It seems to me probable that, whilst the periphery 

 of these cells may be characterized by amceboid movements, the core 

 acts in a more circumscribed way, much as a muscle would. If this is 

 so, the distal and proximal migrations, as well as the expansion of the 

 cell body, ai'e probably manifestations of its amoeboid movements, 

 while its shortening is probably due in the main to the muscle-like con- 

 traction of its central core. Objection might be raised to this com- 

 bination of diff"erent modes of motion, were it not generally admitted 

 that muscular action is, after all, only a more circumscribed form of 

 amreboid movement. 



The presence of a conti-actile axis in the distal retiuular cells is further 

 rendered probable by the fact that in Mysis (Parker, '91, p. 120) an 

 axial core free from pigment has been observed in each distal retinular 

 cell. At the time I first noticed these cores I suspected that they might 

 be the remains of nervous axes, but I now believe there are stronger 

 reasons for suspecting them of being contractile bodies. 



The following table gives by way of summary the periods required 

 for the completion of the various photomechanical changes in the retina 

 of Pala;monetes. 



It is a noteworthy fact, that of these changes those that take place 

 in the light (positive stimulus) are always accomplished more rapidly 

 than the corresponding reversals in the dark. To this statement an 

 apparent exception may be found in the tables on pages 285 and 286, in 

 which are recorded the periods for the contraction and expansion of the 



