44 BRITISH TUN10ATA. 



The mantle (PI. XXIII, figs. 1-3) is delicate and 

 transparent, with the radiating muscles towards the 

 base of the tubes well developed ; the walls of the 

 tubes are rather stout, and are occasionally of a 

 yellowish flesh-colour. 



The structure of the branchial sac (PI. XXIII, figs. 

 4-6, and fig. 36 in text) is very beautiful ; the external 

 surface exhibits with great distinctness the rows of 

 large quadrate openings leading into the pouches of the 

 internal folds; and the inner surface has a rich, delicate, 

 lace-like appearance. The seven longitudinal folds are 

 well arched and are prettily marked or reticulated, 

 chiefly by the membranous bands or rods which stretch 

 along them from end to end, and the primary trans- 

 verse vessels which cross them in a somewhat radiating 

 manner from the ventral to the dorsal margin. There 

 are four or five of these bands, or rods, on each fold ; 

 they are rather wide and have the free border thickened 

 or cord-like. The spiral arrangement of the secondary 

 vessels is very complicated and difficult to trace on 

 account of the multiplicity of the parts ; on the surface 

 between the folds the vessels take an undulatory course 

 more or less in a longitudinal direction, with here and 

 there a tendency to a spiral disposition ; but within 

 the pouches towards the margins of the folds the 

 spirals are much more perfectly defined, though it is 

 difficult even here to trace them with precision ; but it 

 is distinctly seen that the centres of the spirals are 

 arranged in pairs at the bottom of the pouches or 

 within the margins of the folds. The oral lamina is 

 plain, much widened near the mouth, and narrow 

 upwards. There are 8 or 9 large tentacular filaments 

 with smaller ones between them, in all 18 or 20 ; they 

 are sometimes decidedly bipinnate, but most frequently 

 only imperfectly so, being to a great extent merely 

 pinnate. 



The alimentary canal (PI. XXIII, figs. 1-3) forms 

 a single open loop which reaches nearly to the dorsal 

 margin. The liver (PL XXIII), which is of a green 



