70 



BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



The large dorsal eve (figs. 45 and 46) resembles that of Cyclosalpa 

 virgula, except for three points : First, the optic plug is wholly wanting, 

 as is true of all species outside the subgenus Cyclosalpa; second, the 



rod-cells are very irregular and are evi- 

 dently degenerate; third, the position of 

 the eye with reference to the ganglion 

 is different. In Ritteria hexagona the 

 ganglion has rotated forward forty-five 

 degrees, as is shown by the position of 

 the zone of large cells from which the 

 nerves arise (z. in fig. 46). The origi- 

 nally dorsal surface is dorso-anterior. 

 The eye is connected with the originally 

 no. 45. -ritteria hexagona, aggre- anterior part of the originally dorsal sur- 



GATED FORM, DORSAL VIEW OF GANGLION, . r , i i i , i» , 1 l 



eye and accessory eyes, x ioo diam- face [now the dorsal part of the anterior 

 ETER3. from metcau (1893, o. surface], as in all other species. A simi- 



lar rotation is seen in Apsteinia punctata (fig. 57, p. 78), and is still 

 more marked in Thalia (fig. Ill, p. 117). The long axis of the eye in 



ey- 



ty 



CM.. 



K 



..... 



>/ f* 







OS. O.C 



p' 





Fig. 46. 



-Ritteria hexagona, aggregated form, sagittal section of ganglion and eye. 

 diameters. From Melcalf (1893, c). 



X 160 



Ritteria hexagona stands at an angle of 45 degrees to the originally dorso- 

 ventral axis of the ganglion. The same is true in Sal pa maxima and S. 

 fusiformis (fig. 79, p. 92). In the Cyclosalpas, on the other hand, the 

 eye has bent forward until its long axis is nearly parallel to the antero- 

 posterior axis of the ganglion. The Cyclosalpas retain the ganglion 

 in its primitive position, but rotate the eye forward from 160° to 180° 

 in different species. Ritteria hexagona and Apsteinia punctata (fig. 

 57) rotate the ganglion forward 45° and the eye rotates about 140° 

 forward. Thalia democratica rotates the whole ganglion and eye 

 forward nearly 180°. The position of the eye in relation to the body 

 of the salpa is about the same in the aggregated zooids of all species 

 except the Thalias, Pegea, and the Traustedtias, but it is effected 

 in some by the rotation of the eye alone ; in others by the rotation of 

 both eye and ganglion. 



