334 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



The ectoderm has remained attached to the originally dorsal and anterior 

 faces of the ganglion, as it is in other species ; so, in the shifting of the 

 ganglion, it has been carried ventralward, making a loop beneath the 

 ganglion. A comparison of Fig. 14, Plate LV, with Fig. 2, Plate LV, 

 makes this intelligible. The points marked an in each are homologous 

 and were both originally anterior (compare also Fig. 10, Plate LIV). If 

 we bear in mind this change of position it will render the description of 

 the eye itself much clearer. 



The eye is divided into three portions, e'l, e'2 and ex of Figs. 14 and 

 15, Plate LV (see also Fig. 6, Plate LVII). In the anterior of these, ex, 

 the rod cells are on the side toward the ganglion, i. e. are dorsal, and the 

 pigment layer is next the ectoderm, i. e. -away from the center of the 

 ganglion or ventral. In the other two portions the rod cells point away 

 from the ganglion, i. e. are ventral. In one of these two portions (e'l) 

 the rod cells point more forward, while in the other portion (e'2) they 

 point nearly ventralward (compare Figs. 14 and 15, Plate LV, and 

 Fig. 6, Plate LVII). The optic nerve enters the eye between the three 

 regions and is distributed to their rod cells, as seen in Figs. 14 and 15, 

 Plate LV. If, now, Fig. 15 be revolved through an arc of 180 in a plane 

 vertically perpendicular to the paper and it be compared with Fig. 4, 

 Plate LII, we see that we have the following relation between the two, 

 shown by the arrangement of rod and pigment cells and by the manner 

 of innervation. The portion marked ex in one is homologous with the 

 portion marked ex in the other ; the only difference being that in Salpa 

 democratica-mucronata this portion has pushed out a little further from 

 the ganglion. The portions marked e'l and e'2 in Fig. 15, Plate LV, 

 correspond to the single portion marked e' in Fig. 4, Plate LII, i. e., they 

 correspond to the undivided basal portion of the large eye of the chain 

 Salpa cylindrica, and so to the two posterior limbs of the larger eye of 

 Cyclosalpa pinnata, chain form. There is in Salpa democratica-mucro- 

 nata no portion of the eye homologous to the apical portion, e", of the 

 large eye of the chain Salpa cylindrica or Cyclosalpa pinnata. These 

 homologies are deduced from three points : first, the evident shifting that 

 has taken place in the ganglion of Salpa democratica-mucronata ; second, 

 the relative position of the histological elements in the different portions 

 of the eye, and the relation of this position to the three principal axes of 

 the ganglion ; third, the innervation of the eye. 



We see, then, that Salpa democratica-mucronata falls into the same 

 group with Salpa cylindrica, Salpa runcinata-f usiformis, Salpa Africana- 



