M. M. METCALF ON THE EYES AND SUBNEURAL GLAND OF SALPA. 311 



as three processes. In Salpa runcinata-fusiformis, chain form, the cells 

 of the intermediate layer are distinct and show usually a single process 

 toward the rod cells. 



The third layer of the retina (3) is the pigment layer (Fig. 9, p), com- 

 posed of cells so full of pigment granules and so closely massed together 

 that no structure can be made out. A study of their development and a 

 comparison with other species show them to resemble, save for the pig- 

 ment they contain, the cells of the intermediate layer of the retina or 

 the ordinary cells of the ganglion. These pigment cells are arranged in 

 a semicircle enclosing the intermediate cells and the inner ends of the 

 rod cells. 



I did not find the innervation of the eye in the solitary Cyclosalpa 

 pinnata, but in the solitary Salpa democratica-mucronata (Plate LV, Fig. 

 10), the outer, thin-walled ends of the rod cells receive fine fibers that come 

 from the dorsal part of the ganglion, apparently pushing out from the 

 non-cellular core, through the peripheral cellular portion. It is probable 

 that the eye receives fibers also directly from the dorsal cells of the 

 ganglion. The distinctive histological character of the retina is seen 

 from this description to consist of but two kinds of modification of the 

 ordinary ganglion cells : first, the pigmentation of certain cells otherwise 

 unmodified ; second, the more complicated differentiation of the rod cells. 

 No lens is ever present. The eye must be a very efficient light-perceiving 

 organ, but the arrangement of the rod cells, the total absence of any lens 

 and the simple character of the whole organ seem, beyond doubt, to show 

 that it can give no perceptual image. 



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



At a time in the development of the embryo, when the central 

 cavity of the nervous system has just been obliterated, and the central 

 cells of the ganglion are degenerating to form the central, non-cellular 

 core, the cells destined to form the optic ridge push up from the dorsal 

 surface of the ganglion. These cells are arranged, from the first, in a 

 ridge having the characteristic horse-shoe shape, the open end of the 

 horse-shoe being anterior. Salensky 1 describes the embryonic solitary 



1 " Im Nervenganglion kann man nun zwei Theile unterscheiden : einen unteren 

 das eigentliche Ganglion und einen oberen die Anlage der Augen. Letztere setzt 

 sich vom ersteren durch eine Einschniirung ab und verlangert sich nach vorn und 

 hinten in zwei solide Fortsiitze die Anlage der Augen. Die Augen sind durch eine 

 mittlere Briicke mit eiuander verbunden." W. Salensky [15]. 



