CIRRIPEDIA. 281 



The ring is completed below by tbe large suboesophageal ganglion, 

 which gives off the nerves to the trophi, the first pair of cirri- 

 gerous feet, and the adjacent muscles. The second thoracic gan- 

 glion supplies the second pair of feet ; the third ganglion supplies the 

 third pair of feet ; the fourth and fifth pairs of ganglions are approxi- 

 mated to each other ; and the tubular extensile tail or penis receives 

 the two last pairs of nerves. Two branches from the oesophageal 

 ring pass to a small ganglion on either side of the stomach, from 

 which the alimentary canal is supplied. The nervous system is that 

 part of the organisation of the mature animal which most unequivo- 

 cally indicates the province of the Animal Kingdom to which the 

 class Cirripedia belongs ; and it indicates their superiority to the 

 Anellids, and their closer affinity to the crustaceous type by the 

 large size of the confluent ganglions on the abdominal chords. In 

 the degree of approximation of the two chords to each other, the 

 Lepades most resemble the lower isopodous Crustacea, for example, 

 the Talitrus. The neurilemma is stained by a dark brown pigment 

 in the Lepas fascicularis. In Pollicipes 31ifella, the fourth and 

 fifth thoracic ganglia are confluent. In the Balanus thitinnahulum 

 and Coronula diadema all the nerves, save those connected with 

 the brain, radiate from a single great suboesophageal ganglion.* 



The acoustic organs are situated just beneath the basal articulation 

 of the first pair of cirri. Each consists of a sac-like cavity, which 

 incloses the true acoustic vesicle. The orifice of the vesicle is 

 closed by a delicate lid, formed by the expansion of a large nerve, 

 which here abruptly terminates.f Mr. Darwin, to whom we owe 

 the knowledge of this structure, has not found any otolites in the 

 acoustic vesicle, but only groups of yellowish nucleated cells in 

 the pulpy fluid. The same excellent observer is of opinion that 

 the pouches situated in the confluent segments beneath the free 

 part of the outer maxillae are " olfactory ; " their orifices are 

 produced or tubular in many Barnacles, but not in the sessile 

 Cirripeds. 



Although the eyes of the Cirripeds are more or less aborted in 

 their mature state, they retain sufficient susceptibility of light to 

 excite, in the pedunculated species, when a shadow passes over 

 them, retraction of the cirri, and, in the sessile species, a sudden 

 shutting of their opercules ; the same act is caused by the sound or 

 vibration of an approaching footstep ; it indicates that they appre- 

 ciate the atmospheric movements produced by the approximation of 

 the hand, even, according to Dr. Coldstream, when it is not brought 

 nearer the shells than twelve or fourteen inches.J 



* CCXXIV. p. 88. t lb. p. 95. X CCXVI. p. 688. 



