M. M. METCALF ON THE EYES AND SUBNEURAL GLAND OF SALPA. 323 



nerve. There is no organ in Salpa cylindrica corresponding to the 

 posterior pair of small eyes in Cyclosalpa pinnata. 



The structure of the larger eye, which, I believe, corresponds to the 

 large, unpaired dorsal eye of Cyclosalpa pinnata, is shown in Figs. 5 to 7, 

 Plate LII, which represent vertical cross-sections through three regions 

 of the eye. Fig. 5 represents the more posterior section cutting the eye 

 in front of the origin of the optic nerve (compare Fig. 4, Plate LII). The 

 optic nerve is shown on the dorsal side at on ; the rod cells at s' ; the 

 pigment layer at p'. The layer of intermediate cells is mostly wanting, 

 though a few do show here and there. In other words, the posterior cells 

 of the retina have almost all become pigmented. In this region the rod 

 cells are seen to be dorsal and the pigment layer ventral. In the anterior 

 region of the eye this arrangement is reversed, the rod cells being ventral 

 and anterior and the pigment dorsal (Figs. 6 and 7, Plate LII). There is 

 no secondary portion of the eye corresponding to that which lies in the 

 anterior curved portion of the large dorsal eye of Cyclosalpa pinnata, 

 chain form. There are, however, two peculiar small lobes of the eye 

 (a in Figs. 5 and 6, Plate LII). The long axis of the eye is not parallel to 

 the midline of the body, but is quite oblique (Fig. 4, Plate LVI), so that 

 these sections, which are transverse to the long axis of the body, cut the 

 eye obliquely. If it were not for this obliquity the two small lobes would 

 appear in the same section in the region where the relative arrangement 

 of the rod cells and pigment cells of the basal portion is reversed to give 

 the condition found nearer the apex. They are a structure character- 

 istic of this species, not being developed in any other. They cannot, 

 without further study of the development, be homologized with the 

 smaller portion of the dorsal eye of the chain Cyclosalpa pinnata, 

 because of the exactly opposite relative position of the histological 

 elements in the two cases. 



SALPA RUNCINATA-FUSIFORMIS. 



Solitary Form. 



In the eye of Salpa runcinata-f usiformis we find quite marked varia- 

 tions from any of the forms described. In the solitary form we have the 

 typical horseshoe-shaped retina in the usual position, but the develop- 

 ment of retinal tissue at the anterior ends of the two anterior limbs is 

 very great. In this region all the cells of the whole optic ridge are 

 modified to form the retina. Figs. 4, 5 and 6, Plate LIII, represent cross- 



