REPORT ON THE TUNICATA. 133 



bars were still present in the branchial sac, since these vessels are now present in 

 Pharyngodictyon. In the side branch leading to Pharyngodictyon, however, a 

 remarkable degeneration of the branchial sac took place, resulting in the total suppres- 

 sion of the system of fine interstigmatic vessels. This particular change in the structure 

 of the branchial sac appears to be associated with the abyssal zone, since it has taken 

 place independently in four distinct groups of deep-sea Ascidians, viz., Culealus, 

 Fungtdus, Bathyoncus, and Pharyngodictyon, and has not been found in any shallow- 

 water form. 



The second side lirauch from the Polyclinida), tliat leading to the genus 

 Tylobranchion, appears to have left the main axis just aliout the point where the 

 internal longitudinal bars in the branchial sac were disappearing, as Tylobranchion now 

 shows no internal longitudinal liars, but possesses a system of papillae which there is 

 reason to believe are rudimentary connecting ducts, and which sometimes give oflF 

 projections resembling the rudiments of internal longitudinal bars found attached to 

 the free ends of the connecting ducts in some Simple Ascidians.' 



The genus Atopogaster, which may be placed upon a third short twig given off from 

 the axis of the Polyclinidae, differs very slightly from typical members of the family. 

 The only modification of importance which is found, is in the structure of the stomach. 

 The wall of this organ is thrown into a series of more or less distinct transverse folds, 

 a condition never found in the other genera of the family. 



The last side branch from the axis previous to H. is the line leading to Polyclinuvi, 

 a form characterised by the smooth-walled stomach, the twisted intestine, and the 

 pedunculated, laterally-placed post-abdomen.' 



At the point indicated by H. in the tal)le (p. 120) the axis of the Polyclinidae 

 divided into two lines of descent — the one leading to a series of genera, Parascidia, 

 MorcheUioidc'S, Synoicum, Sidnyum, and Morchellium, in which the stomach wall is 

 curiously and irregularly thickened ; and the other to a second series of genera, 

 Fragariuin, Circinalium, Amaroucium, Aplidium, and Psammaplidium, in which the 

 stomach wall is thrown into longitudinal folds. All these genera are really very closely 

 allied, and some species form connecting links between two or more of them.^ A remark- 

 able modification is found in the branchial aperture of some of the forms from both these 

 groups. As a general rule, amongst Compound Ascidians the branchial aperture is six- 

 lobed, but in Parascidia and MorcheUioides in the one group, and in Fragariura and 

 Circincdium in the other, the branchial aperture has become eight-lobed. Psamma- 

 plidium is an interesting new genus derived from the old and well-known genus 

 Aplidium by a modification of the test, which has acquired the property of taking up 



J See this Report, Part II. p. 157. . 



2 See Giard, Arch. d. Zool. cxpdr., torn. i. p. 641 ; and von Drasche, Die Synascidien, etc., p. 23, Wien, 1883. 



" See this Report, Part II. pp. 176 et seq. 



