1206 



TUNICATA. 



liver cells in Boltcnia are elongated coeca, thick, 

 and sometimes deeply divided at the outer 

 extremity, attached by their thin ends, and 

 arranged in eccentrically radiated groups, en- 

 closed in an epithelial membrane, the whole 

 having externally a racemiform appearance. 



The digestive organs of the Tunicata are 

 subject to congenital malposition ; of which M. 

 Savigny has described two remarkable exam- 

 ples, in Cynthia Momus and Phallusia Turcica ; 

 and Mr. MacLeay was inclined to regard as a 

 malformation a peculiar arrangement of the 

 intestinal canal that was presented by the 

 unique specimen of Cystingia Griffithsii de- 

 scribed by him. In a unique specimen of P. 

 Turcica, examined by M. Savigny, the intestine 

 lay to the left instead of the right of the bran- 

 chial sac, and was found bending backwards 

 and embracing the stomach from below, in- 

 stead of bending forwards at some distance 

 from the pylorus, approaching the superior 

 border of the stomach, and then terminating 

 in the rectum. In a specimen of C. Momus, 

 the alimentary canal was also found on the 

 left-hand side ; but, by a very peculiar intro- 

 version, the pharynx was placed at the pos- 

 terior instead of the anterior extremity of the 

 branchial sac. The intestine descended as 

 far as the bottom of the mantle, folded it- 

 self forward, and ascending parallel to itself, 

 terminated opposite to the pharynx ; so that 

 the anus and the cesophageal aperture both 

 opened into the branchial orifice. The ex- 

 ternal communication through the anal orifice 

 existed as usual. Both of these malformed 

 individuals had their ovaries full of eggs, but 

 were not, apparently, in strong health, and 

 were more than usually infested with ento- 

 mostraca. In the catalogue of the Hunterian 

 Museum (vol.i. p\.5.Jig. 2.), a dissection of an 

 Ascidian is figured after a drawing by John 

 Hunter (the original specimen, however, has 

 not been found), in which there is, apparently, 

 an abnormal elongation of the oviduct, which 

 is accompanied by a slight granular line, both 

 lying on a large tapering tube having much 

 the appearance of intestine ; the anus, how- 

 ever, shows itself projecting from the side of 

 this tubular body some way lower down, in 

 its usual place. The oviduct, and, apparently, 

 the accompanying elongated tube, terminate 

 externally at a minute aperture placed in the 

 sulcus between the two projecting terminal 

 orifices of the test. The oviduct, however, 

 barely reaches this aperture ; and its accom- 

 panying granular line terminates still lower 

 down. 



Organs of circulation. In the Ascidladce 

 there are two large vessels or sinuses, the 

 dorsal and ventral, to which the branchial 

 capillaries, on the one hand, and the heart 

 and peri-intestinal cavity, on the other, are 

 intermediate. The circulation is of the mixed 

 or reptilian type ; both sinuses being in con- 

 nection with systemic and respiratory capil- 

 laries, and the blood, consequently, being sent 

 by one impulse both to the system and to the 

 branchiae, and ultimately returning from both 

 by the same channel. We are prevented 



from calling either of these sinuses arterial or 

 venous, on account of the periodic reversal of 

 the circulation, mentioned incidentally above, 

 and more fully detailed hereafter, whereby 

 they are alternately changed from vein to artery 

 and from artery to vein. One of the branchial 

 trunks, terminating at the heart, however, 

 answers to the branchial veins of the Gastero- 

 pods and Bivalves ; the opposite, and often 

 double, trunk may therefore be looked upon 

 as the branchial artery, and is connected with 

 the veins of the body. The Ascidia, like 

 the rest of the Acephala, has but a left or 

 aortic ventricle, and no ventricle at the union 

 of the vena cava and the pulmonary artery. 

 This aortic heart or ventricle is not always 

 easy to be seen. When the branchial sac is 

 simply oblong, it is situated towards its base ; 

 and consequently, when the branchial sac is 

 as long as the body, it is situated towards the 

 base of the mantle ; and when the sac is 

 shorter than the body, it is placed near the 

 centre of the mantle. When the branchial 

 sac is bent upon itself, the heart is situated at 

 the. curve, and then it is always near the 

 middle of the body. In general its position, 

 according to Cuvier, appears to be deter- 

 mined rather by that of the mouth, than that 

 of the rectum ; but M. Milne-Edwards and 

 Van Beneden consider that it follows in its 

 displacement the organs of generation rather 

 than the mouth. The heart in the Tunicata 

 is never traversed by the rectum, as in other 

 Acephala. 



In the Ascidians the form of the heart is 

 oblong, and thin at the two ends, or more 

 or less tubular. Its substance is contractile, 

 but extremely thin and transparent, so that 

 it is scarcely distinguishable in the cavity of 

 its highly pellucid pericardium. Cuvier ob- 

 served that, in the species in which the 

 branchial sac is bent upwards, he was not 

 able clearly to discern a dilatation sufficiently 

 marked to deserve the name of a heart, and 

 was inclined to think that possibly in this 

 case the heart's function was performed by 

 the artery. Here, however, we may notice 

 that specimens preserved in spirit generally 

 afford but very indistinct traces of this organ ; 

 and Cuvier does not appear to have had the 

 opportunity of studying transparent specimens 

 of the living animal, in which the heart can 

 be detected by its pulsating movements. 



In Ascidia intestinalis the heart, which is 

 very long, and extended under the ventral 

 border of the respiratory sac, communicates 

 with the great thoracic sinus by a longitudinal 

 slit situated at a little distance from its an- 

 terior extremity ; and when the peristaltic 

 movements of the heart advance from behind 

 forwards nearly all the blood contained in its 

 cavity passes into this sinus, penetrating the 

 vascular network of the branchial sac, and 

 passing into the dorsal sinus, whence it is 

 spread amongst the viscera, and returns to 

 the posterior extremity of the heart not far 

 from the anus. During this time the heart 

 consequently performs the functions of a 

 branchial ventricle, and the great thoracic 



