ACOUSTIC ORGANS. 95 



When an individual was touched under water, not by a 

 needle, but by a pointed camel-hair brush, it generally 

 withdrew its cirri, but the neighbouring specimens took no 

 notice : when touched by a single hair of the brush, no 

 notice was taken, unless the skin of the orifice leading 

 into the sack was so touched. In these trials, it is of 

 course necessary carefully to avoid intercepting the light. 

 I could not make out that cirripedes perceived odours 

 diffused in the water. 



Acoustic Organs. — These are situated in the same position 

 as in the Lepadidse, namely, in a slight swelling on the sides 

 of the thorax (PI. 25, fig. 1, d') just beneath the basal articu- 

 lation of the first pair of cirri. The orifice in Tubicinella 

 and Xenobalanus is slightly produced, or is tubular; the 

 free part in the former genus projecting ^tlis of an inch. 

 The structure of all the parts is essentially the same as in the 

 Lepadidae, but I think all are proportionally larger. The 

 external membrane of the body is turned inwards at the 

 orifice, as a short flattened tube, which widens considerably 

 (being, in a middle-sized specimen of Coronula, y^ths of an 

 inch in width) before it abruptly terminates. The meatus, 

 as I have called the sack-like cavity which encloses the true 

 acoustic sack or vesicle, is formed of pulpy membrane, and 

 is apparently continuous with the corium of the whole 

 body, but by dissection it can be separated entire. The 

 acoustic vesicle is of various shapes, as we shall immediately 

 see; but in all essential respects it is identical with the 

 same part in the Lepadidae ; it is formed of the same pecu- 

 liar, soft, elastic, brownish, transparent tissue, which seems 

 to be composed of fine, transverse pillars, becoming towards 

 the outside fibrous, and at their inner ends appearing when 

 viewed vertically from above, like hyaline points. In 

 Coronula diadema, I observed On the outside of the acoustic 

 vesicle, some excessively minute bristles, only g^ths of an 

 inch in length, seated on little eminences. I examined 

 carefully the contents of the vesicle in this species, in speci- 

 mens well preserved in spirits, and there was nothing 

 within but a very little, thin, pulpy fluid, and a few 

 yellowish nucleated cells, here and there aggregated into 

 small groups. In Coronula, the flattened acoustic vesicle is 



