ON THE PERIVISCERAL CAVITY OF CIONA. 121 



cardique des autres AscidienSj les communications avec le sac 

 branchial doivent exister k droite et a gauche de ce silloUj a 

 supposer toutefois que ces orifices persistent pendant toute la 

 duree de la vie chez ces Ascidiens" (p. 432). Van Beneden 

 and Julin state that they were able to confirm the existence of 

 the two orifices at the posterior end of the pharynx mentioned 

 by Kupffer, but not his statement that they opened into the 

 atrial cavity : " Le seul point que nous n'ayons pas pu con- 

 firmerj c'est que ces orifices deboucheraient dans les cavites 

 peribranchiales " (loc. cit., p. 432). They consider that these 

 two orifices most probably open into the perivisceral cavity 

 and not into the atrial cavity, in which case water could enter 

 into the perivisceral cavity, as described by Kupffer. 



B. Perivisceral Cavity of the Adult. 



A detailed account of the anatomy and relations of the 

 perivisceral cavity of Ciona has been given by Roule (1, p. 105), 

 so that it will be unnecessary to give along description. The 

 perivisceral cavity is separated from the atrial cavity by a 

 septum or diaphragm attached all round to the body-wall ex- 

 ternally, and internally to the posterior end of the pharynx on 

 either side of the posterior groove which extends between the 

 end of the endostyle and the mouth of the CEsophagus ; the 

 atrial or outer aspect of this septum is lined by the atrial 

 epithelium, and internally it is lined by the lining epithelium 

 of the perivisceral cavity. This internal layer extends on to 

 the oesophagus and rectum, which pierce the septum, and forms 

 numerous folds or mesenteries passing to the various organs 

 contained in the cavity ; the chief of these mesenteries passes 

 to the pericardium, completely surrounding it, and attaching 

 it to the end of the pharynx underneath the posterior groove, 

 the body-wall, and the stomach. The heart, V-shaped, lies in 

 the pericardium, and the vessels going to and from it lie in 

 the mesentery around the pericardium. 



Two points require elucidation : 



(i) Do the openings described by Kupfi'er at the end of the 

 pharynx open into the atrial cavity, as he described, or do they 



