42 SUMMARY OP CURRENT RESEARCHES RELATING TO 



these are so arranged that a long and a small one alternate. A large 

 lacuna extends along the anterior or dorsal surface of each tentacle. 

 The hypogangl ionic tubercle is only the orifice of the vibratile organ ; 

 it is situated in the prebranchial region on the mediodorsal line ; the 

 flat epithelium of the buccal siphon becomes ciliated on the vibratile 

 organ. A ridge runs round the siphon and separates it from the 

 branchia ; it has an uninterrupted groove, which, on the ventral side, is 

 in direct relation with the hypobranchial groove, and on the dorsal 

 raphe forms a projection into the branchial cavity. The anterior lip of 

 this groove has a flat epithelium, while the posterior has a characteristic 

 ciliated epithelium similar to that which invests the two external lips of 

 the hypobranchial grooves. There are no traces of mucous cells in this 

 circumcoronal ridge. 



The fourth chapter is devoted to the branchial cavity, which is first 

 considered as an organ of respiration. It is in the form of an ovoid sac 

 suspended in the peribranchial cavity, and its wall is pierced by 

 thirteen to sixteen rows of stigmata, with about thirty stigmata in each 

 row. The wall of the gill is of a very simple structure, and the blood 

 passes through it in all directions ; there are no traces of vessels, or even 

 of regular lacunse. True plates, which are really folds of the branchial 

 wall, hang down into the branchial cavity ; the author calls them 

 interserial plates, and describes them as hanging down freely into the 

 branchial cavity, which they seem to divide into a series of secondary 

 chambers. Attached to them are medio-dorsal " languettes," the free 

 ends of which form a small platform which carries vibratile cilia on the 

 medio-dorsal line. The transverse bands of fundamental tissue which 

 separate the rows of stigmata are not merely connected with the internal 

 tunic by vascular trabeculse, as in all other Ascidians, but they are 

 directly fused with this tunic on either side of the hypobranchial 

 groove for about a third of their extent. The peribranchial cavity 

 becomes divided into a series of secondary cavities, all of which are 

 open on the cloacal side and end by digitiform culs-de-sac on the side of 

 the endostyle, where they penetrate into the tunic. In the interior of 

 each of the interserial bands there is a pair of muscles which extend side 

 by side through its whole extent. They are connected by numerous 

 anastomoses with the longitudinal muscles of the internal tunic, with the 

 fibres of which their fibres are continuous. The margin of the branchial 

 clefts is invested in a very peculiar epithelium, which is called stigmatic ; 

 the cells are greatly elongated in the direction of the long axis of the 

 stigmata, and each of them has a projecting crest on its long axis ; this 

 crest carries from fifteen to seventeen long vibratile cilia. These 

 stigmatic cells are arranged in rows of sis, and the cells of the same 

 row are of exactly the same length. 



The branchial cavity may also be considered as an organ of degluti- 

 tion. The hypobranchial groove or endostyle extends all along the 

 ventral surface of the branchia, forming a cul-de-sac at its anterior end ; 

 the vibratile epithelium of its lips is continuous with that of the 

 posterior lip of the pericoronal groove. Posteriorly it also ends 

 blindly, and here the epithelium is continued as far as the oesophagus. 

 The epithelium is succeeded by two glandular regions, the first of 

 which contains only one glandular mass, while the second bas two ; 

 the cells at the base of the groove carry very long vibratile flagella. 

 The mucus secreted by the groove is not voided all along the ventral 



