98 



BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



The atrial system (fig. 83) shows a well-developed retractor muscle 

 either united, or almost united, at its base, to body muscle V at its 

 point of branching. This retractor muscle is connected distally with 

 six or seven sphincter muscles, the basal one of which is considerably 

 the stronger. The fibers of the sphincter 

 muscles cross over one another where they join 

 the retractor, as shown in the figure. 



The neural gland is reduced in the aggregated 

 Salpa cylindrica. On the right side of laeval 

 individuals and on the left side of dextral in- 

 dividuals the only remnant of the gland is a 

 funnel-shaped pit in the wall of the branchial 

 chamber, originally the mouth of the duct. 

 The duct itself, with the disk, has wholly dis- 

 appeared. On the other side the disk is want- 

 ing, but the distal portion of the duct is present, 

 as is also its pharyngeo atrial aperture. There is 

 a pair of small-celled outgrowths from the gan- 

 glion at the points where the disks of the neu- 

 ral glands used to lie. (See Metcalf, 1903, c.) 

 The larger, dorsal eye (figs. 86, 87, and 88) 

 is unique among the Salpidae. It has basal 

 and terminal portions of the usual type, though 

 very compactly arranged, as if by a shortening 

 of the antero-posterior axis of the eye. At the 

 point where these two portions join there is a 

 small distinct group of optic cells (a in fig. 88, 

 A and B) on each side, not shown in similar 

 form in any other species studied. The thin- 

 walled ends of these rod-cells is toward the axis of the eye, and their 

 thickened rods are just beneath the ectodermal epithelium of the 

 optic chamber. The homologies of these two groups of rod-cells 



e. ."v 



Fig. 86.— Salpa csthnpbica, 



aggregated zoold, dorsal 

 view of ganglion, eye and 

 ciliated funnel. x 130 di- 

 AMETERS. From Metcalf 

 (1893, c). 



Fig. S7.— Salpa cylindrica, aggregated form, sagittal section of the ganglion, eyes, and the 



BASAL PORTION- OS TI[L CILIATED FOTTOEL. X 98 DIAMETERS. FROM METCALF (1893, c). 



can be determined only by a study of their origin in the developing 

 zooids, a study I have not yet undertaken. 



In the ganglion there is one large accessory eye (figs. 87, ex and 89) 

 in very much the same position it occupies in Salpa rundnata and 



