418 
THE VOYAGE OF H.M.S. CHALLENGER. 
curving ventrally as they go, and keeping within the ventral half of each side. About 
three-fourths of the way back they split up and become lost in the irregular fibres sur- 
rounding the posterior end of the body. 
At the base of the atrial siphon a red spot is seen distinctly through the muscles 
of the mantle. This is the pigmented glandular mass, placed on the extremity of the vas 
deferens, which is sometimes so conspicuous in dona intestinalis. It has been described 
in that species by Heller,^ and more recently by Roule,^ who regards it as a renal organ. 
The branchial sac is very similar to that of Ciona intestinalis, and possesses no note- 
worthy peculiarity. Occasionally, as shown in the figure (PI. XLIX. fig. 13), one or two 
stigmata are interrupted where the delicate horizontal membranes connecting the 
intermediate papillae cross them, but no small transverse vessels extending the length of 
a mesh were found. 
Several Crustacea were present as commensals in this branchial sac. 
^ Untersucliimgen iiber die Tunicaten des Adriatischen Meeres, II. Abtb., Denksch/r. d. k. Akad. d. Wiss. Wien, 
Bd. xxxiv. p. 120, 1875. 
2 Houle, Recberches sur les Ascidies Simples des 06tes de Provence, p. 170. 
