vol. 2, pt. 2.] A TAXONOMIC STUDY OF THE SALPIDAE METCALF. 



25 



form is doubtless therefore primitive. This is also indicated by the 

 fact that in the solitary forms of all Salpidae the eye has this horse- 

 shoe form. 



In later stages of development of the aggregated zooid, the arch of 

 the horseshoe separates from the ganglion (fig. 12) and rises up, the 

 w-hole rudiment of the eye swinging at first upward then forward and 



..J 



D 



Fig. 12.— Cyclosalpa pinnata, aggregated zooid, diagrams showing the forward rota- 

 tion AND TRANSFORMATION IN SHAPE OF THE DORSAL EYE. FROM METCALF AND JOHNSON 



(1905); e, eye; gg, ganglion; r indicates in each case the same region of the eye. 



A, A DORSAL VIEW OF THE GANGLION AND THE EYE OF A SOLITARY SALPA. THE HORSESHOE- 

 SHAPED EYE IS ABOVE THE GANGLION, WITH THE ENDS OF THE HORSESHOE POINTING FORWARD. 

 THE DORSAL EYE OF THE VERY YOUNG AGGREGATED ZOOID HAS THE SAME FORM. 



B, The same seen from the right side. 



C, The ganglion and the eye of an immature aggregated zooid. The eye has now 



ROTATED FORWARD AND UPWARD THROUGH AN ARC OF ABOUT 90°. THAT SURFACE OF THE EYE 

 WHICH WAS DORSAL WHEN THE EYE WAS HORIZONTAL (B) IS NOW ANTERIOR. 



D, The same eye and ganglion as in C, but viewed from behind. The eye is in the form 



OF A FLAT PLATE, THE HOLLOW OF THE HORSESHOE HAVING BECOME FHLED WITH CELLS. 



E, THE EYE AND GANGLION OF AN ADULT SEEN FROM THE RIGHT SIDE. THE EYE HAS SHIFTED 

 90° STILL FURTHER FORWARD AND IS AGAIN HORIZONTAL, BUT WITH ITS FORMER POSTERIOR 

 PORTION ANTERIOR AND ITS FORMER DORSAL SURFACE VENTRAL. 



F, Dorsal view of the same ganglion and eye as in E. The eye is now split posteriorly 



INTO TWO LIMBS, RIGHT AND LEFT, BEING AGAIN HORSESHOE-SHAPED. 



downward, the two ends of the horseshoe remaining attached to the 

 dorsal surface of the ganglion and forming the pivots upon which the 

 eye rotates. By this rotation the originally ventral surface of the 

 rudiment of the eye becomes first posterior then dorsal. Three other 

 changes occur in the developing eye during the gradual completion 

 of this forward rotation. First the space between the limbs of the 

 horseshoe-shaped optic ridge becomes nearly filled by proliferated 



