692 CLASS Xf: 
Doliolum, extending in some Salpw as far as the cesophageal aper- 
ture. The tube is closed posteriorly, but, according to LnucKARrt, 
communicates anteriorly with the longitudinal semicanal. Its walls 
are composed of large nucleated cells, arranged perpendicular to its 
cavity, and LeuckArT suggests that it may perform a secreting 
function. Huxtey has seen it in Salpew, Pyrosomata and certain 
Botryllide, as well as in Cynthia, and, as it seems to be figured 
by Saviany and others, it may perhaps be concluded to be common 
to the Tunicata?.] 
In the Ascidie the intestinal canal usually forms two bends, 
which lie toward each other, and have their convexity facing back- 
wards. The intestinal canal commences at the bottom of the 
branchial cavity, and becomes narrower posteriorly. The stomach 
is sometimes merely indicated by a first expansion of the intestine, 
which is not sharply defined; in other cases its form is elongate and 
cylindrical, as in Didemnum and Botryllus*. ‘The extremity of the 
intestinal canal mounts higher than the commencement of the 
cesophagus, in the direction of the second tubular opening of the 
external integument of the body. The liver les as a stratum of 
glands on the walls of the stomach or the intestinal canal. Salivary 
glands have as little been met with here as in the bivalve molluses. 
The blood-circulation of the tunicata presents the remarkable 
phenomenon, that the direction in which the blood streams from 
the heart is at intervals altogether reversed, so that the heart alter- 
nately drives the blood to the branchiw, and may be called venous, 
and alternately receives the blood from the branchie and, as in the 
rest of the mvertebrates, is arterial. This peculiarity was first 
discovered in 1821 by Van Hassett in Salpa*, but was afterwards 
observed by Lister, Minne Epwarps, VAN BENEDEN and others, 
1 [See Huxtey Phil. Trans. 1851, p. 588. ] 
2 See Savieny Mém. 1. Pl. XX. xx. On the intestinal canal of Ascidia compare 
Cuvier Ascidies, Pl. 1, fig. 5; Catalogue of the Physiol. Series of Comp. Anat. of 
the Museum of the Royal College of Surg. 1. Pl. 5, fig. 1, Phallusia nigra Sav., and 
Van BENEDEN Recherches sur ’ Embryogenie, V Anatomie. et la Physiologie des Ascidies 
simples, Mém. de l’Acad. royale de Belgique, Tome xx. 1846, Pl. 1. fig. 6. 
3 Alg, Konst-en Letterbode 1822, 1. bl. 115, 116 (translated in Ann. des Se. nat. 1. 
pp. 78—81). Afterward MEYEN in his Voyage observed this motion of the blood in 
two opposite directions in Salpa also; Act. Acad. Cws. Leop. Carol. XVI. I. p. 377. 
With this in some degree may be compared, what was observed by J. MUELLER 
(MeckeEt’s Archiv, 1828, s. 22—29) in Nephelis respecting the inconstancy of direction 
in the blood-current, and by E. H. Wesmr (ibid. pp. 399, 400) in young leeches, 
