692 CLASS xi. 



Doliolum, extending in some Salpce as far as the oesophageal aper- 

 ture. The tube is closed posteriorly, but, according to LEUCKART, 

 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. HUXLEY has seen it in Salpce, Pyrosomata and certain 

 Botoryllidm, as well as in Cynthia^ and, as it seems to be figured 

 by SAVIGNY and others, it may perhaps be concluded to be common 

 to the Tumcata l .~\ 



In the AscidioR 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 

 oesophagus, in the direction of the second tubular opening of the 

 external integument of the body. The liver lies 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 molluscs. 



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 branchia3, and may be called venous, 

 and alternately receives the blood from the branchia3 and, as in the 

 rest of the invertebrates, is arterial. This peculiarity was first 

 discovered in 1821 by VAN HASSELT in Salpa 3 , but was afterwards 

 observed by LISTER, MILNE EDWARDS, VAN BENEDEN and others, 



1 [See HUXLEY Phil. Trans. 1851, p. 588.] 



2 See SAVIGNY Mem. n. PI. XX. xxi. On the intestinal canal of Ascidia compare 

 CUVIER Ascidies, PI. i, fig. 5 ; Catalogue of the Physiol. Series of Comp. Anat. of 

 the Museum of the Royal College of Surg. I. PL 5, fig. i, Phallusia nigra SAV., and 

 VAN BENEDEN Recherches sur V Embryogenie, V Anatomic et la Physiologic des Ascidies 

 simples, Mem. de VAcad. royale de Belgique, Tome xx. 1846, PL I. fig. 6. 



3 Alg. Konst-en Letterbode 1822, i. bl. 115, 116 (translated in Ann. des Sc. nat. in. 

 pp. ^g 81). Afterward MEYEN in his Voyage observed this motion of the blood in 

 two opposite directions in Salpa also ; Act. Acad. Goes. Leop. Carol, xvi. i. p. 377- 

 With this in some degree may be compared, what was observed by J. MUELLER 

 (MECKEL'S Archiv, 1828, s. 22 29) in Nephelis respecting the inconstancy of direction 

 in the blood- current, and by E. H. WEBER (ibid. pp. 399, 400) in young leeches. 



