Miscellaneous, 107 



II. — The heart in Ascidia villosa and Cynthia moras is formed, 

 as in the Compound Ascidians, by a diverticulum of the enteric 

 cavity, which becomes isolated at an early period when the peri- 

 branchial sacs are in process of development. This diverticulum 

 becomes a little closed sac, a certain portion of the wall of which is 

 forthwith invaginated, thus producing a double cavity : the inner 

 one is the cardiac cavity, which communicates by the cleft of 

 invagination with the hremal spaces ; the outer chamber is the peri- 

 cardium, which is completely closed and does not contain a drop of 

 blood ; it represents a portion of the archenteron. 



III. — All along the cardiac cleft, and applied to it in the manner 

 of an obturator, is seen another sac with very delicate epithelial 

 walls. At the time of the appearance of the first branchial clefts in 

 Ascidia villosa, this sac still has a wide opening into the enteric 

 cavity from which it is derived, while its other extremity gradually 

 elongates and moulds itself round the alimentary tract after the 

 fashion of a mesentery. 



In its origin, disposition, and relations to the heart this sac is 

 absolutely identical with the epicardium, which hitherto was known 

 only in the Compound Ascidians. 



In Cynthia morns the epicardium appears as two great prolonga- 

 tions of the peribronchial sacs, and thus recalls with great exactness 

 the arrangement which I have described in the Botryllidae. 



The mode of formation of this aperture is precisely as described 

 long ago by Krohn and Kovalewsky and since observed by all 

 ascidiologists ; but the point on which I desire to insist is the endo- 

 dermal origin of the peribranchial cavity. I therefore do not share 

 the view of Mctschnikoff and Kovalewsky, who have maintained 

 that this cavity is due to two ectodermal invaginations, which 

 gradually surround the cavity of the enteron. These two learned 

 naturalists, whose endeavours to study the transparent embryo were 

 evidently impeded by the egg-membranes, were unable to make out 

 precisely the earliest processes, and observed only the stage in which 

 the ectoderm is seen already invaginated. The examination of 

 larvae of all ages, still enclosed in the peribranchial cavity and cut 

 into thin sections, is the only method that enables us to decide tho 

 question with accuracy. 



It is important to determine the origin of the peribranchial 

 cavity in Simple Ascidians, on account of the conclusions with 

 regard to blastogenesis in the Botryllidae which have been quite 

 recently deduced therefrom by a Norwegian ascidiologist named 

 Hjort*. We know that in Compound Ascidians the branchio- 

 intestinal cavity of each bud is produced by the proliferation of the 

 outer peribranchial wall of the parent ascidiozoid, which is of endo- 

 dermal origin in the larva as well as in the bud, as was shown by 

 me in a former paper f . Hjort, without making a study of the 

 Botryllid larva, has applied to it, with regard to the origin of the 



* ' Anatomischer Anzeiger,' Band x. no. 7. 

 t ' Annales des Sciences naturelles,' 1892. 



