vol-. 2, ft. 2.] A TAXONOMIC STUDY OF THE SALPIDAE METCALF. 117 



e.z 



Fig. 110.— Thalia democratica, aggre 

 gated form, dorsal view of gan 



GLION AND EYES. X 362 DIAMETERS 



From Metcalf (1893, c). 



probable homologies appear when we realize that the anterior rota- 

 tion of the eye, observed in the development of Cyclosalpa pinnata 

 (fig. 12, p. 25) occurs in the same way in Thalia democratica but has 

 gone about one hundred and twenty degrees further, the ganglion 

 itself sharing in this rotation, as is clearly shown by the development 

 of the eye and ganglion in the buds and by the arrangement in the 

 adult of the ectodermal epithelium over 

 the originally dorso-anterior face of the 

 ganglion, which is now ventral. If, in 

 imagination, we rotate the ganglion back 

 one hundred and twenty degrees to a 

 position comparable to that in, say, Salpa 

 fusiformis (fig. 79, p. 92), we see that in 

 position relative to the ganglion, and in 

 the relative position of the rod and pig- 

 ment cells, the anterior portion of the 

 eye in Thalia democratica(ex) is compar- 

 able to the large accessory eye (ex) in 

 the ganglion of Salpa fusiformis. Its 

 pigment layer, present in Thalia, is want- 

 ing in Salpa. The two posterior por- 

 tions of the Thalia eye (e'l and e'2) represent, then, the large dorsal 

 eye of the true Salpae, or rather the proximal portion of this eye, 

 as is shown by the position of rod-cells and pigment cells. The 

 division of this eye into two parts, right and left, is a reversion to 

 the condition seen in Cyclosalpa pinnata (figs. 7 and 8, pi. 2), in which 

 the proximal portion of the large dorsal eye is divided into distinct 



right and left limbs. The inner- 

 vation of the portions of the eye 

 in Thalia agrees with this inter- 

 pretation. The anterior portion 

 (e") of the large dorsal eye, found 

 in Cyclosalpa pinnata and in the 

 other species thus far described 

 in this paper, is lacking in Thalia, 

 as is also the optic plug (e'"). 

 < 300 diameters. That portion of the eye which is 

 marked e'2 is seen to be oriented 

 slightly differently from the portion marked e'l. The meaning of 

 this difference in orientation is not clear. It may have to do with 

 the position of the zooid in the chain. 



Neither in the fully formed aggregated individuals of Thalia 

 democratica, nor in the course of their development, are there any 

 traces of outgrowths from the ganglion, or of either the chambers 



Fig. ill.— Thalia democratica, aggregated 

 zoOid; longitudinal vertical section through 

 the oanglion and eyes 

 From Mf.tcalf (1893, c). 



