322 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



SALPA CYLINDRICA. 



The eye of Salpa cylindrica, in both the solitary and chain forms, 

 presents more marked differences from the conditions found in Cyclo- 

 salpa pinnata. In the solitary form the only point of difference is that 

 the rod cells of the posterior curved part of the horseshoe are directed 

 ventralward instead of forward, and the pigment layer is dorsal (Fig. 1, 

 Plate LII). In the two anterior limbs of the eye the condition is the 

 same as in the corresponding regions in Cyclosalpa pinnata (Fig. 2, 

 Plate LII). 



In the eyes of the chain form there are very marked features of 

 difference from Cyclosalpa pinnata. Fig. 4, Plate LVI, gives a surface 

 view, and Fig. 4, Plate LII, shows the general structure. The larger eye 

 is on the dorsal surface of the ganglion, projecting a little, but not far 

 beyond its anterior face. There is a single large optic nerve entering 

 the posterior part of the eye. The latter figure shows that there are two 

 eyes: one larger (e r , e"), plainly seen in surface view (compare Fig. 4, 

 Plate LVI); the other, smaller (ex), placed behind the former on the 

 dorsal surface of the ganglion (Fig. 4, Plate LII). A vertical cross- 

 section of the ganglion, through this smaller eye, is shown in Fig. 8, 

 Plate LII, and a more enlarged drawing is given in Fig. 9 of the same 

 plate. The appearance shown in longitudinal section is the same (Fig. 4, 

 Plate LII, ex), indicating that the organ is circular. This eye corre- 

 sponds, I believe, to the dorsal pair of smaller eyes seen in Cyclosalpa 

 pinnata. It agrees with the latter in histological structure and in posi- 

 tion, only that the two have fused together in Salpa cylindrica across the 

 middle line behind the optic nerve. Notice that the thin-walled, nucle- 

 ated ends of the rod cells of the small eye point toward the core of the 

 brain, as do the rod cells of the small dorsal eyes of Cyclosalpa pinnata. 

 The fusion of the two small dorsal eyes into one in Salpa cylindrica 

 corresponds to the more compact form of the larger eye and the fusion 

 of the two optic nerves into the single nerve found in the latter species. 

 For convenience of description I refer the structure of the eyes in the 

 different species to that found in Cyclosalpa pinnata. I believe, how- 

 ever, that the eye of that species is not primitive, but is the most removed 

 from the ancestral condition. We start, then, in our study with the more 

 modified forms and pass to the less modified. Fig. 9, Plate LII, shows 

 the innervation of the smaller eye. The fibers that come to it arise from 

 the non-cellular core of the brain, in close connection with the optic 



