88 BALAN1U.E. 



Nervous System. 



It has been shown in my former volume, that in Lepas 

 and in some other genera of the Lepadidge, there are six 

 main ganglions ; one supra- oesophageal, and five infra- 

 cesophageal, or thoracic. In Pollicipes, however, there are 

 only four thoracic ganglions, the last ganglion supplying the 

 three posterior pairs of cirri with nerves, whereas in the 

 other genera, the last ganglion supplies only the fifth and 

 sixth pairs of cirri. In this genus, moreover, the lateral 

 fusion of the ganglions has been so complete, that there is 

 no evidence of their having been formed by the union of 

 two. Amongst sessile cirripedes, we discover evidence of 

 much higher concentration even than in Pollicipes. My 

 chief examination has been confined to Coronula diadema, 

 and to Bala?ius tintinnabulum : and in these genera we find 

 (and the fact appears to me highly remarkable) as high a 

 degree of concentration in the infra-cesophageal ganglion 

 as in any decapod Crustacean, for instance, as in Maia, 

 judging from the figure given by Milne Edwards ; for all 

 the nerves, with the exception of those connected with the 

 supra-cesophageal ganglions, radiate from a single great 

 ganglion.* The nervous system is, moreover, otherwise 

 complicated. 



To begin with Coronula dladema the great infra-cesopha- 

 geal ganglion (PL 27, fig. 1, a) is seated nearly opposite to 

 the anterior margin of the second pair of cirri, which are homo- 

 logous with the first pair of legs in the decapod Crustaceans. 

 This ganglion shows no trace of any longitudinal medial 

 suture ; its shape is hardly discoverable, for it is formed 

 by the union of eleven principal pairs of nerves, besides 



* It must, however, be observed that, according to Mr. Dana, there is 

 in certain suctorial Entoniostracans, as in Caligus, only one infra-oesophageal 

 ganglion. Mr. Dana speaks of this as resulting from reduction. In Cirripedes, 

 from the gradation which may be observed from Lepas through Pollicipes into 

 Balanus, the ganglions are certainly not reduced but concentrated. In Van de 

 Hoeven's figure of the nervous system in Limulus, there is seen to be no chain 

 of thoracic ganglions ; all the nerves rising from the circa- oesophageal collar; 

 but this, on the other hand, seems hardly developed into a ganglion. 



