^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 i 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 different 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 

 «anal, which, at both extremities, passes 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 effusion of the blood into the parenchyma 

 of the body and its return into the veins without 

 the intervention of capillaries, or in general with- 

 out walled canals, has been maintained recently, es- 

 pecially by Milne Edwards (Observ. et exper. sur 

 la circul. chez les MoUusques, Comp. Rend. XX. 

 -1845, p. 261), and by Valenciennes '^Nouv. observ. 

 sur la constit. de I'appareil de la circul. chez les 

 MoUusques, Ibid. p. 750). Their observations 

 ■were not limited to Salpa, and the Ascidiae, but 

 were extended upon Oslrea, Pinna, Maclra, Ve- 

 nus, Cardiwm and Solen. See also Ann. d. Sc. 

 Hat. III. 1845, p. 289, 30", or Froriep^s neue Not. 

 I4os. 732, 733, 743. 



Milne Edwards is about to publish an extended 

 •work, on the circulation with the iloUusca. He has 

 figured from his beautiful injections the partly la- 

 cunal circulatory system of Pinna ; see Ann. d. Sc. 

 Hat. VIII. 1847, p. 77, PI. IV. 



i For the blood of Ptiallusia, Cynthia, and Ano- 

 donta, see Wagner, Zur vergleich. Physiol, d. 

 Blutes lift. I. p. 20, II. p. 40. The blood-corpuscles 

 of the Naiades have always appeared to me of an 

 irregular form ; and they run together when placed 

 in a watch-glass. This is probably due to the fibrin 

 cementing them together. When treated with ace- 

 iic acid they become separated again, their contour 

 becomes very clear and almost imperceptible, and 

 a hitherto invisible nucleus is seen. 



1 See Cuvier, loc. cit. p. 10, tig. 2, .^9, &c. Ac- 

 cording to Meyen (loc. cit. p. 375, PI. XX VIII. fig. 

 1, d.) the heart of Salpa mucronata has two con- 

 strictions ; and, according to Eschricht, that of 

 Salpa cordiformis is divided into four chambers 

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



2 Meyen (loc. cit. p. 376) has denied the pres- 

 ence of a pericardium with Salpa ; but Cuvier (loc. 

 cit. p. 10), Savisrny (loc. cit. p. 127), and Delle 

 CAwje (Uescriz. &c. III. p. 43, Tav. LXXVIU.) 

 affirm the contrary. 



3 The direction of these blood-currents in the 

 body of Salpa is satisfactorily shown by the de- 

 scriptions and figures of Quay and Gaimard (loc. 

 cit.) and especially of Delle Cliiaje (Descriz. &c.) 



Sars (Faun. litt. &c. p. 66), has also observed, 

 with Salpa runcinata, that the blood beyond the 

 aorta and vena cava, circulates in wall-less pas- 

 sages. 



4 This remarkable alteration of the blood-currents 

 which is possible only with a valveless heart, has 

 been observed and described by different observers 

 in a conformable manner. Before the heart chang- 

 es the direction of its contractions it remains still 

 for a short time, and this slackens the course of the 

 blood-currents in the body a Uttle, before they re- 

 ceive an impulse in the opposite direction ; see 

 Van Hasselt (Ann. d. Sc. Nat. III. 1824, p. 78). 

 Eschscholtz {Muller^s translation of the annual 

 report of the Swedish Academy upon the progress of 

 Natural History, &c., 1825, p. 94), Quay and Gai- 

 7nard (loc. cit. p. 559, or Isis. 1S36, p. Ill), and 

 Delle Chiaje (Descriz. &c. III. p. 43). 



5 For the heart and blood-system of the Asci- 

 diae, see especially, Milne Edwards (Snr les Asci- 

 dies composees loc. cit. p. 4), who has indicated the 

 presence of the heart in Phallusia and Clave- 

 lina, as well as in Polyclinum, Botryllus, Di- 

 demnum, Pyroaoma, (fc. 



