ASCIDIA AFFINLS. Io7 



of oyster fisheries. The specimens have usually a 

 soiled appearance, are much infested by parasitic 

 zoophytes, and are generally united at the base into 

 clusters, the base being considerably prolonged into 

 a sort of irregular, flat pedicle. 



The test (PL XV, fig. 8) is rather thin and devoid 

 of the echinations common to the group to which the 

 species belongs, except towards the tubes which are 

 roughened and spinous. 



The mantle is well supplied with interwoven 

 muscular fibres, the marginal and transverse ones 

 predominating, and is of a pale brownish colour when 

 preserved in spirit ; the tubes are wide, well produced, 

 and strongly folded longitudinally. 



The branchial sac (PL XVII I, fig. 9) is minutely 

 plicated and considerably elongated, with the mouth 

 opening into it near to the bottom ; the primary vessels 

 are pretty -regularly disposed, and do not vary greatly 

 in size; and the ciliated discs which they bear are 

 large, ovate, and placed in pairs between the longi- 

 tudinal bars ; the latter are stout, not much elevated, 

 with the suspended membrane wide ; the papillary 

 membranes are moderately developed and have the 

 free margins much thickened and projecting very 

 slightly beyond the rods like short truncate papilla, 

 one at each primary vessel. 



The oral lamina (PL XIX, fig. 12) is wide, and for 

 the greater part smooth, the ribs being confined to the 

 base, and it terminates rather abruptly at the right 

 side of the mouth ; the margin is entire. The left oral 

 appendages are 7 or 8 in number ; they are well-deve- 

 loped, pectinated, triangular leaflets placed diagonally 

 and each having from one to three points or teeth. The 

 branchial tubercle (PL XX, fig. 10) is large and trans- 

 versely oval ; it is much convoluted and has the inferior 

 margin indented. There are between 30 and -40 ten- 

 tacular filaments ; they are alternately large and small, 

 and are not cro \vded; their bases are a little widened 

 and marked with opaque white. 



