324 JOHNS HOPKINS UNIVEESITY MORPHOLOGICAL MONOGRAPHS. 



sections of three regions of the eye of the solitary Salpa runcinata- 

 fusiformis. The more posterior (Fig. 4) cuts the optic ridge through the 

 posterior curved portion just at the origin of the anterior limbs. We see 

 the core of the ridge at cr and the retina above it, e. In this species, as 

 in Salpa cylindrica, the pigment layer of this portion of the eye is 

 dorsal, p ; next to this comes the layer of intermediate cells, -/ ; ventral 

 to this again are the rod cells. These are not definitely columnar and 

 arranged in a single layer, but they are irregular in shape, have irregu- 

 larly thickened walls and are arranged in an agglomerate mass. The 

 rod cells are elongated in a dorso-ventral direction, suggesting vaguely 

 the columnar structure of the corresponding cells of the retina of Salpa 

 cylindrica or Cyclosalpa pinnata. Further forward, in the anterior limbs 

 of the eye, the relative position of pigment cells and rod cells is changed, 

 the former being on the outer, the latter on the inner face of the optic 

 ridge (Fig. 6, Plate LIII). This corresponds to the position in other 

 species. Fig. 3, Plate LIII, a longitudinal section through one of the 

 limbs of the eye, shows the great development of retinal tissue, eq, at the 

 anterior end of the eye. Fig. 6, Plate LIII, a cross-section of this region, 

 gives a better conception. The anterior limbs are seen on the dorsal 

 side, e ; between the anterior limbs of the eye there is a great mass of 

 rod cells, eq. They are best called rod cells because of their undoubted 

 relation to the typically developed rod cells of other species, though in 

 this species they do not have the ordinary structure of rod cells. This 

 ventral extension of the anterior ends of the two anterior limbs of the 

 eye is a peculiar feature not represented in the solitary form of any other 

 species studied. The condition of the rod cells seems to indicate that the 

 retina is degenerate, but rod cells of a very similar character are found 

 in several species yet to be described, and it will be well to defer any 

 discussion of the point till we can gain a comprehensive view of all such 

 species. 



Between the ectoderm and tunic, in the region above the ganglion 

 and eye, there is a large cavity which in life would be full of sea-water. 

 This would act as a cushion, and must be a very efficient protection for 

 the ganglion and eye. Fig. 3, Plate LIII, shows the relation of this 

 cavity, ts, to the adjacent organs. 



Chain Form. 



The eye of the chain form of Salpa runcinata-fusiformis (Plate LVI, 

 Fig. 6) shows certain very interesting characters. In shape it is elon- 



