MOLLUSCA—THE PALLIAL COMPLEX 



77 



placed auricle, and the aorta which runs forward from the chamber lying behind the 

 auricle have pulled these chambers round in such a way that the flow of blood can 

 have a straight course {cf. diagram, Fig. 74), the ventricle will then come to lie 

 in front of the auricle. Indeed, the pericardivmi (with the ventricle and auricle) 

 has actually twisted round 180°. In this twisting it has been followed by the 

 kidney, which is connected with it by the reno-pericardial aperture, so that the 

 latter organ no longer lies to the right but to the left of the pericardium, the aper- 

 ture of the urinary duct remaining at its former place. The whole reno-pericardial 

 complex, as compared with its typical position in the Piolmonatn, is quite reversed. 

 This reversal is characteristic of TestaceUa. 



It is, further, noteworthy that, in TestaceUa, the floor of the pulmonary cavity 

 becomes invaginated anteriorly into the body below it to form a large air sac. The 

 walls of this sac are not supplied witli blood vessels, and it seems to serve merely as 

 a reservoir of air. In many Testacellidcc the reno-pericardial complex hangs down in 

 the shape of a sac into this air sac from the roof of the pulmonary cavity. 



In the ]''a<jLnididai and the Oiicidia the arrangement of the organs, originally 

 belonging to the pallial complex, deviates still further from the type. A shell is 



Fig. 74.— Diagrams to illustrate the changes of position in the pallial organs of Daude- 

 bardia and TestaceUa (adapted from figures by Plate). Mantle organs drawn a.s in Fig. 73. A, 

 Daudebardia rufa ; B, Hypothetical stage, tlie pallial comple.K of A twisted round 00° ; 0, 

 TestaceUa. l, Respiratory aperture; 1', kidney; 3, ureter or urinary duct; 4, reno-pericardial 

 aperture (renal funnel) ; 5, ventricle ; (J, auricle ; 7, aorta ; S, pulmonary vein ; 0, pulmonary 

 vascular network. 



wanting in the adult and a mantle also ; and the mantle- or pulmonary cavity 

 seems in consequence to have atrophied. The jjericardium lies posteriorly to 

 the right, sunk into the integument, the ventricle lying, as in TestaceUa, in front 

 of the auricle. Respiration takes place j)rincipally through the skin ; in the amphib- 

 ious Oiicidia it is assisted by dorsal papillre. In Vaginulns, the urinary duct joins 

 the proctodaeum to form a cloaca which somewhat widens at the point of junction, 

 and opens externally at the posterior part of the body. The same is the case in 

 most Oiicidia, but in Oncidium cclticum, the urinary duct and the rectum emerge 

 separately, but one close to the other, at the posterior end of the body. Close to 

 these apertures lies, in all cases, the female genital aperture ; the male aperture, 

 however, lies anteriorly to the right below the tentacle. 



The cloaca just mentioned, which is filled with air, has given rise to interesting 

 discussions. From its wall there rise into the lumen closely packed folds, which 

 may also be continued along the posterior portion of the urinary duct. The cloaca 

 has therefore been considered by some to be a rudimentary pulmonary cavity, into 

 which the urinary duct and the rectum open. The present writer holds the opinion, 



