M. M. METCALF ON THE EYES AND SUBNEURAL GLAND OF SALPA. 300 



greater or less extent in the different species. The eye of the chain-salpa 

 is not, however, in all cases a simple structure with a single origin, as is 

 the case in the solitary form, but in several species distinct eyes are 

 developed, or new portions of distinct origin are added to that part of the 

 eye which is homologous with the eye of the solitary form. 



First I will describe the adult eye of the solitary Cyclosalpa pinnata 

 and its development, as the type to which to refer. Then I will describe 

 the adult eyes of the chain form of the same species and trace their 

 development. With this as a foundation we will pass to the study of the 

 other species. 



THE ANATOMY OF THE EYE OF CYCLOSALPA PINNATA, SOLITARY FORM. 



On the dorsal surface of the ganglion of the solitary form of Cyclo- 

 salpa pinnata there is a ridge shaped like a horse-shoe, with the open end 

 of the horse-shoe anterior. (Compare Fig. 7, Plate LVI, which represents 

 the eye, ganglion, and adjacent parts of the solitary Salpa scutigera-con- 

 federata. The condition in the solitary Cyclosalpa pinnata is essentially 

 the same.) This ridge, like the ganglion, is composed of a cellular peri- 

 pheral portion and a non-cellular core; the cellular portion of the one 

 being continuous with that of the other, while the core of the ridge is 

 continuous with the central, non-cellular portion of the ganglion (Plate 

 LI, Fig. 8). The height of the ridge above the ganglion is a little greater 

 than its width. On the anterior face of the posterior part of the ridge 

 and on the inner faces of its two anterior limbs, the cells are modified to 

 form the retina of the eye (Plate LI, Fig. 8). The eye, then, in every part 

 faces toward the mid-dorsal point of the brain. The retina is formed 

 from the more dorsal cells of the inner half of the optic ridge (that half 

 toward the mid-dorsal point of the brain). The cells of the outer half of 

 the ridge and of the ventral part of the inner half exactly resemble the 

 cells of the ganglion. In Cyclosalpa pinnata, these cells, which are not 

 shown in the figure of the eye of Salpa scutigera-confederata, would 

 appear in surface view as a thin layer outside the pigment. 



As seen in Plate LI, Fig. 8, the ectoderm lies on the outer and dorsal 

 surfaces of the optic ridge, but does not descend to the ganglion between 

 the anterior limbs of the ridge. Between the dorsal surface of the gang- 

 lion, the optic ridge and the ectoderm there is a space (oc) which is a 

 blood lacuna. This space, the optic chamber, is nearly shut off from the 

 space in which the ganglion lies by a membrane (z'} which stretches 

 from the ganglion, at the base of the optic ridge, out on all sides to the 



