M. M. METCALF ON THE EYES AND SUBNEURAL GLAND OF SALPA. 353 



amount of variation. This is greater in the chain forms than in the 

 solitary forms. Salpa cylindrica, solitary form, shows the same struc- 

 ture of the lateral tubes as Cyclosalpa pinnata, but there are no ventro- 

 lateral outgrowths from the brain (Fig. 1, Plate LII). In the chain form 

 the right tube is reduced to a mere funnel-shaped pit in the wall of the 

 branchial chamber (Plate LII, Fig. 11, d'). The left is more developed, 

 but does not extend to the surface of the brain. It arises from the wall 

 of the branchial chamber at the level of the posterior surface of the 

 brain and runs forward and toward the middle line, ending blindly 

 beneath the center of the brain, without any enlargement comparable 

 to the hollow disks found in Cyclosalpa pinnata (Plate LII, Figs. 10, 11 

 and 12, d and do). There are present in the chain form one pair of very 

 slight ventro-lateral outgrowths from the brain, containing only the 

 smaller kind of ganglion cells (Plate LII, Fig. 8, 6). Salpa scutigera- 

 confederata, chain form, shows the lateral tubes and disks typically 

 developed, and a single pair of large-celled ventro-lateral outgrowths 

 from the brain (Plate LV, Figs. 3, 4 and 5). I have not sectioned the 

 solitary form. The chain form of Salpa bicaudata closely resembles that 

 of Salpa scutigera-confederata in the structure of the subneural gland 

 and adjacent organs (Plate LV, Figs. 8 and 9). In the chain form of 

 Salpa democratica-mucronata there is no trace of lateral ducts or out- 

 growths from the brain. This is perhaps associated with the twisting 

 of the ganglion described in the section on the eye. In the solitary form, 

 on the other hand, the ducts are present in much the condition found in 

 Cyclosalpa pinnata. Plate LV, Fig. 10, shows a longitudinal section 

 cutting one of the hollow disks. There are no outgrowths from the 

 brain. In the solitary form of Salpa runcinata-fusiformis the tubes and 

 hollow disks are present and are unusually large (see Fig. 4, Plate LIII). 

 Their epithelial lining is for the most part apparently lost, the walls of 

 the tubes and of the greater part of the disks being formed seemingly 

 only by the membrane continuous with the basal membrane of the 

 wall of the branchial chamber. 1 



There is in the chain form of this species an antero-ventral out- 

 growth from the ganglion, shown in Fig. 9, Plate LIII : we shall see 

 later that this is important. The chain form of Salpa costata- Title >iii 

 shows a different condition of these organs. The wall of the branchial 



1 It is probable that those portions of the tubes which appear in section to have 

 no epithelial lining have in reality a lining of pavement epithelium not visible in 

 section. 



