98 BRITISH TUNICATA. 



is much corrugated, particularly in S. mamittaris, on 

 wliicli parasitic growths are very frequent. It is 

 strongly adherent to the mantle or inner tunic. The 

 internal surface is very smooth and has almost a silvery- 

 white appearance, varied in parts with a rosy hue. It 

 readily divides into two layers ; the inner layer is 

 delicate and liable to be left adhering to the mantle. 



The mantle is likewise comparatively thick and 

 tough, and is so opaque that the arrangement of 

 the muscular fibres is not readily distinguished. In 

 S. mamillaris, however, delicate, longitudinal fibres 

 can be traced passing from above downwards. The 

 viscera are completely obscured in both species. In 

 S. tuberosa the mantle is of a pale flesh colour; in 

 S. mamillaris it is of a neutral green hue suffused with 

 scarlet in the region of the tubes. The former animal 

 is elongated and pyriform ; the latter is irregularly 

 oval transversely. 



The aerating; surface of the branchial sac is com- 



o 



posed of a minute rectilinear reticulation of primary 

 or transverse and minute secondary vessels, and on 

 each side of the oral lamina there are four wide folds 

 which stretch from one end of the sac to the other. 

 The network is in other respects as simple as it is in 

 AscAdia venosa. The folds all terminate below in the 

 vicinity of the mouth, drawn as it were into a focus ; 

 but as the mouth is placed considerably up the ventral 

 side, all the folds are much arched backwards and 

 downwards in conformity with the line of the endostyle, 

 those nearest to that organ being most curved; the 

 lower portion of the folds, in fact, have to ascend to 

 reach their terminal position by the side of the mouth. 



The longitudinal bars are represented in Styela by 

 numerous, fine, ribbon-like membranes which extend 

 the whole length of the branchial sac ; they are placed 

 at some little distance apart, and are as numerous on 

 the folds as they are between them. 



The oral lamina is a broad, smooth membrane, 

 passing from the branchial tubercle to the right side 



