98 



distributed, but are most numerous and extend nearest to 

 the base of the eye stalk, on the upper side wliicb is 

 turned towards the light (fig. 29, on left side). 



The name iris has been given (fig. 29, /.) to the zone 

 bounding the cornea where most pigment is present. The 

 cells of the iris become rapidly less tall, the nucleus 

 becomes more central, and then suddenly all pigment 

 is lost, and they pass into the cubical, transparent cells of 

 the cornea (fig. 29, Cor.). A thin layer of cuticle covers 

 the epithelial cells of the eye stalk, the iris and the cornea. 



The function of the iris seems to be, like that of the 

 pigmented sides of the eye vesicle and stalk, to prevent 

 lateral rays of light penetrating to the retina. 



The cells of the cornea Avere discovered by Patten (35) 

 to be pentagonal or hexagonal in surface view, hour-glass 

 shaped in section, quite free from pigment, and to have a 

 curious interlocking or notched appearance on the edges, 

 so as to give the cornea a much firmer and more rigid 

 structure as a membrane. This appearance was described 

 by Schreiner (37) as being due to intercellular substance 

 and to fixation alone. 



From transverse and longitudinal sections through 

 these cells fixed in Mann's Fluid, Flemming's Eeagemc 

 and Osmic Acid, I am of opinion that they do possess 

 processes on all sides which probably connect them 

 together, as in the so-called " Prickle Epithelium." This 

 occurs also in the epidermal cells of the eye stalk (fig. 35). 



The connective tissue which makes up the rest of the 

 eye stalk is a continuation of the same from the mantle, 

 and is similar in structure. There are numerous lacunae 

 with blood corpuscles, and the rest consists of 

 almost structureless, homogeneous connective tissue with 

 scattered nuclei and a few muscle fibres (fig. 29, Op. m.), 

 which later are present in considerable quantity in the 



