OLFACTORY AND ACOUSTIC SACKS. 53 



into a deep sack lined by pulpy corium, and closed at the 

 bottom. The outer integument is inflected inwards, (hence 

 periodically moulted,) and becoming of excessive tennity, 

 runs to near the bottom of the sack, where it ends in an 

 open tube: so excessively thin is this inflected membrane, 

 that, until examining Anelasma, I was not quite certain 

 that I was right in believing that the outer integument 

 did not extend over the whole bottom. I several times 

 saw a nerve of considerable size entering and blending 

 into a pulpy layer at the bottom of the sack of corium ; 

 but I failed in tracing to which of the three pair of 

 nerves, springing from the front end of the infra -oesopha- 

 geal ganglion, it joined. I can hardly avoid concluding, 

 that this closed sack, with its naked bottom, is an organ 

 of sense ; and, considering that the outer maxillae serve 

 to carry the prey entangled by the cirri towards the 

 maxillae and manclibles, the position seems so admirably 

 adapted for an olfactory organ, whereby the animal could 

 at once perceive the nature of any floating object thus 

 caught, that I have ventured provisionally to designate 

 the two orifices and sacks as olfactory. 



Acoustic (?) Organs. — A little way beneath the basal 

 articulation of the first cirrus (PI. IX, fig. 4d, and PL IV, 

 fig. 2e), on each side, there maybe seen a slight swelling, 

 and on the under side of this, a transverse slit-like orifice, 

 o'oth of an inch in length in Conchoderma, but often only 

 half that size. In Ibla this orifice is seated lower down 

 (PI. IV, fig. 8a, e), between the bases of the first and 

 second cirri, which are here far apart : in Alepas cornuta 

 it is placed rather nearer to the adductor scutorum 

 muscle, namely, beneath the manclibles. The orifice leads 

 into a rather deep and wide meatus ; the external integu- 

 ment is turned in for a short distance, Aviclening a little, 

 and then ends abruptly. The meatus, enlarging upwards, 

 is lined by thick pulpy corium, and is closed at the upper 

 end; from its summit is suspended a flattened sack of 

 singular and different shapes in the different genera. 

 This, the so-called acoustic sack of Conchoderma virgata, 



