TUNIC AT A. 



1207 



sinus is a kind of pulmonary artery. But 

 when this state of things has lasted some 

 minutes, the direction of the peristaltic move- 

 ment of the heart is inverted, and the blood, 

 instead of traversing the branchial network 

 from below upwards as previously, moves 

 from above downwards, and passes from the 

 great thoracic sinus into the heart. The 

 latter is then an aortic ventricle, and the sinus 

 a branchial vein or aortic auricle. In a very 

 fresh and uninjured individual, of Cynthia am- 

 pulla, Van Beneden counted 45 contractions 

 in one direction ; and then, after a rest during 

 the space of two pulsations, he counted 160 to 

 170 in the other, the pulsations being about 

 70 per minute. 



In injecting the vascular system of the 

 simple Ascidians, M. Delle Chiaje thought he 

 found certain valvules so disposed as to hinder 

 the return of the blood from the aorta into 

 the cavity of the heart, or from passing again 

 from the heart into the vessels through which 

 it had arrived there. But by careful observa- 

 tions on living specimens, both Milne Edwards 

 and Van Beneden have established the fact, 

 that, as in the Botryllida:, the Salpidcc, and 

 the other Ascidian families, the blood of the 

 Asciclifidfs, after having flowed for some time 

 in one direction, traverses the same circle in 

 an opposite direction; a condition that would 

 be impossible were any valvular hindrances to 

 return currents of the blood present. 



In Chelyosoma* , the heart is very distinctly 

 seen in the animal when dissected ; it lies near 

 the resophagus, and has two distinct chambers. 

 The aorta rising from its anterior part is a 

 stoutish vessel, and at first lies close to the in- 

 testine : it afterwards runs in the space within 

 the intestinal loop, ultimately breaking up into 

 largish branches, distributed on every side. 

 The ramifications divide some 4? or 5 times, 

 and terminate somewhat abruptly, the extre- 

 mities appearing as if closed. Throughout 

 the surrounding generative organs there is a 

 very fine network of vessels, but whether 

 they are arteries, veins, or gland-ducts is un- 

 decided. A largish vessel running along the 

 left side of the stomach and duodenum ap- 

 pears to return the blood to the branchial sac. 



Mr. MacLeay describes the heart of Cys- 

 tingia as being large, ovoidal, and of a lobular 

 appearance ; and having four vertical, lateral 

 openings, capable of considerable dilatation. 



In Cynthia ampulla the heart is placed a 

 little within the great intestinal loop, and 

 near the middle of the body : it is fixed on an 

 oblong vesicle, enclosing calcareous concre- 

 tions. This vesicle is situated exterior to 

 and above the first or principal bend of the 

 intestine. Its colour is a greenish yellow, 

 and it has apparently no aperture, or commu- 

 nication with other organs. The heart itself 

 is a slightly bent tube, with very elastic walls : 

 it has two openings ; a single large aperture 

 on one part, and opposite to it three ves- 



* Described by Esehricht (Royal Danish Trans. 

 vol. ix. p. 12.), to whom we are indebted fur much 

 valuable information, both with respect to this 

 genus and the Salpians. 



sels that carry oft' the blood in different di- 

 rections.* 



" The circulation of the Ascidians," says 

 Van Beneden, " differs but little from that of 

 i\\eBryozoa; and is transitional between that of 

 the Polypes and of the Molluscs. If we re- 

 move the heart of the Ascidian, the disposi- 

 tion of parts is very similar, and the simpli- 

 fication of an apparatus cannot more visibly 

 take place. The Ascidia is but a digestive 

 canal suspended in the midst of external mem- 

 branes, with a liquid moving in the peri- 

 intestinal space. A colourless liquid (blood) 

 occupies this cavity ; but it is only in the 

 branchial network and tentacles that it can 

 be said to be contained in vessels. All 

 around the intestinal tube this fluid is alter- 

 nately moved from right to left, and vice versa. 

 In the vessels composing the vascular net- 

 work, and in the respiratory tentacles, the 

 same movement of the nutrient fluid takes 

 place. This blood contains somewhat regular 

 globules, white as the containing liquid, that 

 indicate the course of the fluid. In some in- 

 dividuals the blood is yellowish. M. Milne- 

 Edwards has observed an Ascidia with red 

 blood."-]- Mr. Lister observed that in a sessile 

 Ascidian half an inch long, the blood-globules 

 were about the same size as those of the 

 minute Perophora, viz. from '00025 to '0002 

 inch in diameter. 



Van Beneden, to whom we are indebted for 

 so much information with regard to simple 

 Ascidians in general and Cynthia ampulla in 

 particular, has observed in living specimens of 

 this species, that the blood is distributed to 

 each branchial trunk at the same time, and 

 with the same direction, ascending and de- 

 scending alternately. According to this, the 

 heart, contracting in one direction, sends 

 blood towards the branchise, filling all the 

 vessels at one time, and recalls it by contract- 

 ing in an opposite direction. That a series of 

 contractions in one direction, during a certain 

 time, may take place without engorgement, 

 there must be a direct communication between 

 thebranchias and the peri-intestinal cavity; and 

 this is afforded, according to Van Benedcu, 

 by the respiratory tentacles, see p. 1202. 



Seen under the microscope, the contractile 

 tissue of the heart affords no trace of mus- 

 cular fibre. The contractility remains some 

 time after the removal of this organ from the 

 body ; and that without being irritated. 



Nervous system. A single ganglion, oval, 

 soft, consisting of a saclike neurilemma, enclos- 

 ing nerve-cells or neurine, placed in the sub- 

 stance of the mantle, and between its two tubu- 

 lar orifices, gives off' four branches, two of the 

 branches forming a loop around either tube, 

 and other lesser filaments distributed about 



* In the Physiological Series of the Hunterian 

 Museum there is a highly illustrative dissection 

 of ,C. tulerculata (No. 898." B.), prepared by I'rof. 

 Owen, that beautifully exhibits the heart and its 

 pericardium, the branchial vessels, partially in 

 jected, and other organs. 



f Recherches Zoologiques faites pendant un 

 Voyage en Sicile. Comptes Eeudus, 23 Nov. IS i 1. 



4 H 4 



