ASCIDIA. 71 



The oral lamina is well developed in all the Aseidise. 

 It is usually a wide plane membrane extending along 

 the ventral margin of the branchial sac from one ex- 

 tremity to the other, passing on the right of the oral 

 orifice in those species in which the sac is prolonged 

 backwards ; in those with the sac not so prolonged it 

 terminates at the right side of the mouth. In many 

 of the species the oral lamina is transversely ribbed ; 

 the ribs are strongly developed in A. mentula, [J. 

 Alderi, A. aculeata, etc.,] and in these species they pass 

 beyond the margin as fine points, giving to the lamina 

 a pectinated appearance ; the function of this organ is 

 to convey the food to the mouth (Plate XIX). 



The endostyle extends the whole length of the 

 branchial sac, and is in connection with two mem- 

 branous folds which form a longitudinal groove and 



O O 



within which it seems to be embedded. It is com- 

 posed of two flattened rods of a rigid, opaque, yellow 

 substance. The folds are in connection with the 

 anterior cord or collar, a narrow cord-like fold of the 

 lining membrane which encircles the entrance to 

 the branchial sac, and which is likewise in connection 

 with the oral lamina. Below, the folds of the endo- 

 style are also connected to the oral lamina by a similar 

 cord-like fold. 



The circle of tentacular filaments within the base of 

 the incurrent tube is always situated a little above the 

 entrance to the branchial sac (Plate III, fig. 1). The 

 filaments vary in size and number in the different 

 species in the w T hole of the genus, but are always 

 linear and pointed or slightly conical. 



The branchial tubercle, that curious and enigmatical 

 organ situated immediately in front of the upper 

 extremity of the oral lamina, at the entrance of the 

 branchial sac, is composed of two folds of the lining 

 membrane pressed close together, and convoluted more 

 or less in the various species so that it usually assumes 

 the form of a rounded or oval, depressed, compact, 

 tubercular swelling (Plate XX). 



