96 BALANID.E. 



elongated, with a somewhat sinuous, but not very irregular 

 margin (PI. 27, fig. 4), and is without any ridges on the 

 surface ; its neck or orifice projects at right angles to the 

 elongated portion, which stands obliquely to the tubular 

 orifice of the meatus. In a moderately-sized specimen of 

 Coronula diadema, the elongated portion of the acoustic 

 vesicle was -^ths of an inch in length. In Tubicinella, the 

 acoustic vesicle is heart-shaped, with the neck attached to 

 its broader end ; and the surface is covered by zig-zag 

 ridges. In Balanus tintinnabulum (fig. 3), the acoustic 

 vesicle is almost square at the lower end, with the neck 

 placed at one of the upper corners ; on the external surface, 

 there is an oblique prominent ridge or fold, which sends off 

 downwards another ridge ; its length, in a large individual, 

 was T^ths of an inch. 



In all these cases, the acoustic vesicle is mainly attached 

 by its neck, to the upper end of the sack-like meatus; 

 but there is likewise a layer of soft, pulpy, cellular matter, 

 slightly connecting that side of the vesicle which is opposite 

 to the neck, with the walls of the meatus or outer sack. 

 The mouth or orifice of the vesicle is closed by a delicate 

 lid or diaphragm, which can easily be separated ; and this 

 diaphragm is formed by the expansion of a large nerve, 

 which here abruptly terminates. In a very large specimen 

 of Coronula diadema I clearly made out the existence of 

 this nerve, and traced its course for some distance from the 

 point where the summit of the meatus and the neck of the 

 vesicle are joined together ; the nerve first runs posteriorly, 

 and then turns inwards and doubles back or anteriorly; and I 

 clearly followed it to the antero-lateral sides of the upper- 

 most end of the stomach, where it seemed to enter a gan- 

 glion, so that I unfortunately cut it off, but found only a 

 slight plexus, with the cut off nerve apparently running 

 onwards with nearly the same diameter. The diameter is 

 great, fully equalling, in its widest part, that of the circa- 

 cesophageal chord ; but it is very much flattened, and so has 

 not nearly so much bulk as that nerve. Before it reached 

 the stomach, it gave off one branch, which ran towards the 

 mouth. The only nerves which, from their size, could, I 

 think, be continuous with this from the acoustic sack, are 



