§ 192. THE ACEPHALA. 207 



which are connected by no capillary net-work except that situated in the 

 respiratory organs. The blood leaving the open ends of the arteries passes 

 into the interstices [Lacunae) of the parenchyma of the body; thence it is 

 taken up by the open mouths of the venous radicles.'-" 



The Blood is colorless and contains many pale, granular globules, which 

 are indistinctly nucleated. '^> 



§ 192. 



With Salpa, the circulatory system is composed of two main trunks, one 

 upon the dorsal, and the other upon the ventral median line. At the ante- 

 rior extremity of the body these trunks connect by two arcuate vessels ; 

 and at the posterior extremity by a single slightly-dilated canal situated 

 directly in front of the intestinal nucleus. This last-mentioned canal is 

 divided into several chambers by two or three constrictions, and, from its 

 rhythmical contractions, may be regarded as a heart. *^> It is surrounded 

 with a delicate pericardium, <-> and by its pulsations the blood is thrown across 

 the walls of the body in diiferent ways,*'^* thus forming extra -vascular cur- 

 rents. But it will here be observed that the heart, thus forcing the blood 

 alternately in one direction and then in another, will regularly change the 

 arterial into a venous current, and vice versa.'-^^ 



With the Ascidiae, this system is equally feebly developed. The blood 

 passes for the most part out of the vessels into the lacunae which often con- 

 sist of ramified canals resembling vessels. The Heart is always present, 

 and is surrounded with a very thin pericardium. It consists of a long 

 canal, which, at both extremities, pa.sses into a vessel which lies loop-like 

 between the vascular sac and the intestine at the lower part of the cavity of 

 the body.*''^ Its pulsations quite resemble the peristaltic movements of the 



1 This efifusion of the blood into the parenchyma -' Meijen (loo. cit. p. 376) has denied the pres- 

 of the body and its return into the veins without ence of a pericardium with 6'a/;;a ; but Cuticr (loc. 

 the intervention of capillaries, or in general with- cit. p. 10), Savigny (loc. cit. p. 127), and Delle 

 out walled canals, has been maintained recently, es- CAjo/e (l)escriz. &c. III. p. 43, Tav. LXXVIII.) 

 pecially by Milne Edwards (Observ. et exper. sur affirm the contrary. 



la circul. cliez les Mollusques, Comp. Rend. XX. 3 The direction of these blood-currents in the 

 lS-15, p. 261), andby Fa;e7icJennes(Nouv. observ. body of Sa//)a is satisfactorily shown by the de- 

 sur la constit. de I'appareil de la circul. chez les scriptions and figures of Q,uoij and Gaimard (loc. 

 Mollusques, Ibid. p. 7-50). Their observations cit.) and especially of Delle Chiaje (Descriz. &c.) 

 were not limited to Salpa, and the Ascidiae, but Sars (Faun. litt. &c. p. 66), has also observed 

 were extended upon Ostrea, Pinna, Mantra, Ve- with Salpa runcinata, that the blood beyond tlie 

 nus, Cardium and Solen. See also Ann. d. Sc. aorta and vena cava, circulates in wall-less pas- 

 Nat. III. 1845, p. 2S9, 307, or Froriep^s neueNot. sages. 



Nos. 732, 733, 743. * This remarkable alteration of the blood-currents 



Milne Edwards is about to publish an extended which is possible only with a valveless heart, has 



work on the circulation with the MoUusca. He has been observed and <k'scribed by different observers 



figured from his beautiful injections tlie partly la- in a conformable marmer. Before the heart chang- 



cunal circulatory system of Pinna ; see Ann. d. Sc. es the du-ection of its contractions it remajns still 



Nat. VIII. 1847, p. 77, PI. IV. for a short time, and tliis slackens the course of the 



2 For the blood of Phallusia, Cynthia, and Ano- blood-currents In the body a little, before they re- 

 donta, see fVag-ner, Zur vergleich. Physiol, d. ceive an impulse in the opposite direction; see 

 Blutes lift. I. p. 20, II. p. 40. The blood-corpuscles ?^an Hasselt (Ann. d. Sc. Nat. III. 1824, p. 78). 

 of the Naiades have always appeared to me of an Eschscholtz {Miiller^s translation of the annual 

 irregular form ; and they run together when placed report of the Swedish Academy upon the progress of 

 in a watch-glass. This is probably due to the tibrin Natural History, &c., 1825, p. 94), Quoy and Uai- 

 cementing them together. When treated with ace- mard (loc. cit. p. 559, or Isis. 1836, p. Ill), and 

 tic acid tliey become separated again, their contour Delle Chiaje (Uescriz. &c. III. p. 43). 

 becomes very clear and almost imperceptible, and 5 For the heart and blood-system of the Asci- 

 a hitherto invisible nucleus is seen. diae, see especially, Milne Edwards (Sur k-s Asci- 



1 See Cuvier, loc. cit. p. 10, fig. 2, ,9, &c. Ac- dies composees loc. cit. p. 4), who has indicated the 



cording to 3/eycn (luc. cit. p. 375, PI. XXVIII. fig. presence of the heart in Phallusia and Clave- 



1, d.) the heart of Salpa mucronata has two con- Una, as well as in Polyclinum, Botryllus, Di- 



strictions ; and, according to Eschricht, that of demnum, Pyrosoma, ifC. 

 Salpa cordiformis is divided into four chambers 

 (loc. cit. p. 26, fig. 8, a). 



