Regeneration in Conipoinnl Eyes of Crustacea 1 71 



Before closing the description of the normal eye mention should 

 be made of another point. In young individuals in each of the 

 species examined in this series of experiments there is present a 

 growth zone in the ommatidial region. From this zone the num- 

 ber of ommatidia is increased as the animal grows older and 

 increases in size. In longitudinal sections cut in a horizontal 

 plane this zone is apparent on the inner edge of the eye as a narrow 

 band of elongated cells situated above the basement membrane 

 and between it and the completely developed ommatidia. In 

 some instances partially differentiated ommatidia can be recog- 

 nized in this growth zone. This zone has been mentioned by 

 Parker and others. It is briefly described in this connection now 

 because it is probable that in some instances ommatidia that have 

 apparently regenerated have in reality developed from the growth 

 zone. 



The terminology used in the above description and in the dis- 

 cussion of the regenerating eye is that used by Parker. The 

 series of optic ganglia described by him as occupying the eye stalk, 

 however, have been referred to in this paper as different divisions 

 of a single ganglion, it being thought that the matter could be 

 treated with less confusion in this way. Also, the distal portion 

 of the eye stalk, called the retina by Parker, is referred to here as 

 the ommatidial portion. It is composed of a large number of 

 individual ommatidia and the more general use of the term, 

 retina, does not imply all the structures composing the ommatidia. 

 The structure of the rhabdom as described in the present paper 

 is not in full accord with Parker's description. 



While the above description applies especially to Palaemonetes 

 it is sufficiently accurate for the other forms described in this work 

 to serve all purposes. The most marked differences in the struc- 

 tures of the eye of Palaemonetes and of the other forms used are as 

 follows : In Crangon and in crayfish the eye stalk is much shorter 

 and proportionately greater in diameter than in Palaemonetes. 

 Also, in Crangon some small glands are found located just below 

 the basement membrane. Hermit crabs have long slender eye 

 stalks similar to those of Palaemonetes except that at the base and 

 occupying the dorsal inner edge there is a small pointed squame 



