322 BULLETIN OF THE 



wards, under the stomach, and that the opposed walls of the pro- 

 longation become applied to one another, coalesce, and then become 

 perforated.* At any rate, the mid-atrium is now surrounded by a 

 membranous wall, continuous on all sides with the lining of the lat- 

 eral atria, and applied superiorly and anteriorly against the stomach 

 and oesophagus, posteriorly and inferiorly against the external tunic, 

 but not touching either of these parts, except for a small space on tin; 

 floor of its chamber, where it becomes united with the external tunic 

 to allow of the formation of the atrial aperture. This aperture is 

 situated on the neural side of the body, in front of the posterior end, 

 which is chiefly occupied by the genitalia ; but as development goes 

 on the mid-atrium increases, disproportionately, and encroaches upon 

 the other organs, upwards and forwards, in such a manner that its 

 anterior wall invests the whole posterior and lateral faces of the gas- 

 trointestinal division of the alimentary canal The facts which 



I have detailed are exceedingly important for the comprehension of as- 

 cidian structure in general." This account of the form and connections 

 of the atrial chamber applies to those zooids of Pyrcsoma which are 

 produced by budding, as well as to the egg-embryo, but according to 

 the observations of Kowalevsky the diverticula which give rise to the 

 lateral atria of the bud-zooid are derived, not as in the egg-embryo, 

 from the outer tunic, but from the branchial sac, or inner tunic, and 

 the cavities are therefore diverticula from the cavity of the latter. t 



The atrium of Salpa, when first observed (Fig. 22, 15), was com- 

 posed of two broad lateral atria within the body cavity, one on each 

 side of the branchial sac, and a very small mid-atrium (i) ; so that we 

 have all the essential parts of this structure, occupying similar posi- 

 tions to those of Pyrosoma, and bearing almost identical relations to 

 the surrounding parts. The lateral atria do not, however, as in most 

 Tunicata, remain connected with the mid-atrium, and unite with the 

 wall of the branchial sac to form the branchial slits, but soon be- 

 come entirely separated (Fig. 23,/), and the two walls of each unite so 

 as to form a broad solid sheet of tissue, which soon splits up to form 

 the muscular bands of the branchial sac, of which there are six in the 

 solitary form, and five in the chain-sal pa. 



* This conjecture is fully proved by the observations of Kowalevsky. 



t Compare also the manner in which the atrium is formed in the bnd-zooids of 

 Amauricium and Dideniinum. Ueber die Knospung der Ascidien. von A. Kowalevsky. 

 Archiv. fur Mik. Anat. X ,iS74, p. 441. 



