326 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



Beside the larger eye there is, in the chain form of Salpa runcinata- 

 fusiformis, a smaller collection of cells just dorsal to the point of origin 

 of the optic nerve, which have a structure closely resembling the 

 structure of the rods in many species. Fig. 7, a vertical longitudinal 

 section, and Fig. 9, a cross-section of the ganglion, show the structure of 

 these cells (ex). Like the rod cells of the solitary form of this species, 

 they have no regular shape, but the characteristic thickening of their 

 cell walls is present. As will be seen by comparison with forms yet to 

 be described, they must be regarded as rod cells which are either degen- 

 erate or have not attained to the typical structure. Their position, 

 dorsal and posterior to the point of origin of the optic nerve, indicates 

 that they are homologous with the smaller eye found in the chain Salpa 

 cylindrica, and so with the dorsal pair of small eyes found in Cyclosalpa 

 pinnata, chain form. 



SALPA AFRICANA-MAXIMA. 



The shape of the eye of the solitary form resembles that found in 

 the solitary Salpa runcinata-fusiformis. (I have been unable to study 

 sections of the solitary form.) In histological structure the large eye of 

 the chain form agrees very closely with that of the chain form of the 

 last species (Fig. 1, Plate LIV). There is a mass of imperfect rod cells 

 in the dorsal part of the ganglion corresponding closely in shape, in 

 position and in the character of its component cells to the similar 

 structure found in Salpa runcinata-fusiformis. The only noticeable 

 difference between the eyes of the two species is in the shape of the 

 larger eye in the chain forms. In Salpa africana-maxima this is merely 

 globular or slighly ovoid, while in Salpa runcinata-fusiformis it is elon- 

 gated ovoid. The close resemblance between the eyes of these two 

 species indicates the closest relationship. 



SALPA HEXAGONA. 



The Fish Commission collections contain no specimens of the solitary 

 form of this species. The principal eye of the chain form much resem- 

 bles the larger eye of the chain Salpa runcinata-fusiformis in shape, 

 relative position of histological elements and innervation. (Compare 

 Fig. 5, Plate LVII, with Fig. 6, Plate LVI, and Figs. 13 and 14, Plate LII, 

 with Figs. 7 and 8, Plate LIII.) The histological character of the rod 

 cells resembles more closely that of the imperfect rod cells of the small 



