M. M. METCALF ON THE EYES AND SUBNEUEAL GLAND OF SALPA. 317 



reagents that had left apparently unimpaired the pigment granules in 

 the pigment cells of the large dorsal eye. 



The structure of the dorsal pair of small eyes is the same as that of 

 the posterior pair. In this case, however, the rod cells, which are 

 arranged in a hemisphere, have their nucleated ends pointing toward 

 the center of the ganglion and their thick-walled ends abutting on the 

 pigment layer of the large dorsal eye (Plate XLIX, Fig. 8). A com- 

 parison of Fig. 8, Plate XLIX, with Figs. 2 and 3, Plate LI, which are 

 vertical transverse sections (a little oblique), will show the exact position 

 of these eyes. The right one lies at the base of the right optic nerve, on 

 the right side of and a little in front of it. The left one is situated in 

 a corresponding position on the left side. The pigment layer of the 

 unpaired dorsal eye is separated from the rod cells of each small dorsal 

 eye by the delicate neural membrane that intervenes (Plate XLIX, 

 Fig. 8). This may or may not prevent the small dorsal eyes function- 

 ing as light-perceiving organs in connection with the pigment of the 

 large dorsal eye. It indicates that the small dorsal eyes cannot be 

 regarded as parts of the larger unpaired eye. 



THE DEVELOPMENT OF THE EYES OF CYCLOSALPA PINNATA, CHAIN FORM. 



Dorsal Unpaired Eye. 



The early stages of development of this eye in the chain form are 

 almost identical with those in the solitary form. See Figs. 1 to 3, Plate 

 XLVII, which represent a series of vertical transverse sections through 

 the ganglion at the time of the earliest appearance of the rudiment 

 of the eye. Fig. 1 portrays the more anterior section ; Fig. 3 the more 

 posterior. The rudimentary optic ridge is designated by e. It first 

 appears at a time when the central cells of the ganglion are commencing 

 to degenerate. It has, from the first, the characteristic horseshoe shape 

 seen in the eye of the solitary cyclosalpa. The first two sections cut the 

 two anterior limbs of the ridge. (Compare with Fig. 5, Plate LI, which 

 represents an anterior section of the developing eye of the solitary form.) 

 The third section cuts through the posterior curved portion of the horse- 

 shoe. (Compare with Fig. 6, Plate LI, a section through the posterior 

 part of the developing eye of the solitary form.) The horseshoe-shaped 

 arrangement of the cells of the rudiment of the eye is very soon lost, but 

 it is always present for a short time. I have examined serial sections of 

 the ganglia of more than one hundred individuals of about the age repre- 



