M. M. METCALF ON THE EYES AND SUBNEURAL GLAND OF SALPA. 333 



brought to lie in a direction oblique to the long axis of the body. The 

 innervation of the eye is the same as in the last species, except that the 

 optic nerve forms a long stalk by which the eye is elevated above the 

 ganglion. Fig. 7, Plate LV, shows this stalk and one lobe of the eye. 

 The section is vertical and nearly longitudinal. Fig. 6 shows a section 

 through the base of the two lobes of the eye and the optic nerve between 

 them. In Salpa bicaudata there are no smaller eyes, nor any indication 

 of structures homologous to them. 



Above the eye in the chain form of this species there is a decided 

 thickening of the tunic, forming a cushion, or pad, that must be a very 

 effective protection for the eye and ganglion. 



I have before referred to the nucleus-like bodies in the thick-walled 

 ends of the rod cells of Cyclosalpa pinnata. In Salpa bicaudata similar 

 bodies are seen in the center of the rod cells ; the true nuclei of the cells 

 lying near the periphery (see Fig. 6, Plate LV). In this species they 

 are unusually large. They stain more deeply than the protoplasm and 

 less deeply than the cell walls. They are indicated in Fig. G merely in 

 outline, the deeper staining not being represented. 



SALPA DEMOCRATICA-MUCRONATA. 



In Salpa democratica-mucronata we have decided points of difference 

 from any other species. The eye of the solitary form (Fig. 7, Plate LVII) 

 closely resembles that of Salpa cylindrica, as a comparison of Figs. 10, 

 11 and 12, Plate LV, with Figs. 1, 2 and 3, Plate LII, will show. The 

 chief point of difference is that in the former there are no intermediate 

 cells of the retina to be distinguished. There is also a slight difference 

 in shape, since in Salpa democratica-mucronata there are three distinct 

 swellings of the rod cell layer of the retina, as shown in Plate LVII, 

 Fig. 7. One of these is posterior on the median line, the other two on the 

 two anterior horns of the horseshoe. 



In the chain forms of all other species the larger eye is situated on 

 the dorsal or antero-dorsal face of the ganglion. In the chain Salpa 

 democratica-mucronata it is placed on the an tero- ventral face of the 

 ganglion (Plate LVII, Fig. 6, a dorsal view). The form and structure of 

 the eye and the arrangement of the ectodermal optic sheath show that 

 this peculiarity of position is due to a shifting of the whole ganglion, by 

 which the originally dorsal face has become antero- ventral. The eye 

 then arises from what was originally the dorsal face of the ganglion. 



