200 Mary Isahelle Steele 



the hermit crabs than in either of the other forms. The only 

 structure in the eye of the hermit crab which apparently does not 

 regenerate perfectly is the basement membrane. This membrane, 

 however, is but slightly developed in the normal eye. It is not 

 strange, therefore, that it appears imperfect in the regenerated eye. 



The regenerated eyes of the hermit crabs in these experiments 

 have developed from a level below the basement membrane. In 

 every case they present a clearer and more normal appearance than 

 the regenerated eye of either Palaemonetes or Crangon. This is 

 largely due to the fact that in the eye of the hermit crab there are 

 no shapeless masses of old pigment scattered among the regener- 

 ated tissues as is usually the case in Crangon and Palaemonetes. 

 The absence of the yellow accessory pigment cells in the eye of the 

 hermit crab also tends to give to the ommatidia a distinct and 

 orderly arrangement. The absence of this accessory pigment is, 

 however, not due to incomplete regeneration. The normal eye of 

 the hermit crab, unlike that of many Decapods, contains no 

 accessory pigment cells. These pigment cells are very abundant 

 in the eyes of Crangon and Palaemonetes and tend to make the 

 ommatidia less clear. 



The most significant difference between Palaemonetes and 

 Crangon seems to be in the rate of regeneration after similar 

 injury. External appearances would indicate that Crangon 

 regenerates more rapidly than Palaemonetes. But a comparative 

 study of the section shows that the reverse is true. 



In almost every individual in the series of twenty Crangon 

 operated upon the injury was slight. The wound healed rapidly, 

 the animals moulted frequently and externally there was every 

 indication that regeneration was rapidly taking place. An exam- 

 ination of the sections, however, shows that in none of them has 

 there been any considerable regeneration. On the other hand, 

 in each case much of the old injured tissue remained in a semi- 

 broken down condition. 



Sections of the eye represented in Fig. 8 show that one part of the 

 eye had not been injured below the level of the outer retinular 

 cells. The proximal ends of the retinulae still remain intact 

 although thirty-one days have elapsed since the operation. A 



