M. M. METCALF ON THE EYES AND SUBNEURAL GLAND OF SALPA. 331 



periphery of the ganglion. In the former, the nuclei contain many 

 small chromatin granules, and no very large nucleolus. The other cells 

 of the ganglion are of two sorts, the one sort small, with small nuclei. 

 These are utterly different from the cells we are discussing. The other 

 kind of ganglion cells is larger, about equal in size to the rod cells of 

 the eye. They have the same sized nuclei, but in these the chromatin is 

 nearly all collected into a large nucleolus, giving a decidedly different 

 appearance from the nuclei of the rod cells. Besides this their proto- 

 plasm reacts much more strongly with haematoxylin, giving a deep 

 stain, while the protoplasm of the rod cells stains very weakly. These 

 characters, and especially the great thickness of their cell walls, distin- 

 guish the rod cells of the eye and of the two lateral masses in the gan- 

 glion from any others of the nerve cells. We must, then, regard these 

 two lateral masses of rod-like cells that are found in the ganglion as 

 imperfect or degenerate eyes, bearing the same relation to the larger eye 

 of this species as the smaller dorsal eyes of the chain Cyclosalpa pinnata 

 do to the large, unpaired eye of that species. These structures just 

 described form a connecting link between the smaller eyes found in 

 Cyclosalpa pinnata and Salpa cylindrica, which are undoubted optic 

 organs, and other structures found in Salpa hexagona, 1 Salpa costata- 

 Tillesii, and Salpa cordiformis-zonaria, which are so different from the 

 typical eye that one would not readily recognize their true character. 

 These structures cannot function as optic organs, and when I apply to 

 them the word eye I mean merely that they are homologous to struc- 

 tures in other species that are undoubted eyes, and that the same sort of 

 histological modification which has produced them from the cells of the 

 ganglion has produced the rod cells of the larger dorsal eye of Salpa 

 scutigera-con f ederata. 



The manner of innervation of the dorsal eye of Salpa scutigera-con- 

 federata is very suggestive when compared with that of the correspond- 

 ing eye in the species thus far described. In this species the optic nerve 

 passes up from the ganglion and enters the eye at its mid-ventral point 

 (Fig. 2, Plate LV). This reminds one strongly of the condition in the 

 very young chain Cyclosalpa pinnata (Fig. 7, Plate XLVII), where the eye 

 has the form of a thickened disk close pressed to the ectoderm, and the 



1 The lateral outgrowths from the ganglion of the chain Salpa hexagona are 

 similar in structure and position to the two lateral masses of rod cells in Salpa scuti- 

 gera-conf ederata and seem clearly to be homologous with them (compare Fig. 5, 

 Plate LV, with Fig. 15, Plate LII). 



