INTRODUCTION. Ixxxiii 



rated or toothed aloug the edge. In this genus the entire 

 dorsal surface of the branch is covered by the vibracula, 

 and the movements of the setae are synchronous ; they act 

 together with perfect regularity — the whole company on 

 a branch smnging to and fro at the same moment, and 

 as if under a common impulse (see page 58). We can 

 hardly doubt that there must be some intercommunica- 

 tion between the nerve-centres of the individual vibracula, 

 on which these combined movements depend ; but so far 

 the synchronism has attracted very little attention*, and 

 we have no observations that throw any further light 

 upon it. 



The setse attain their highest development in the family 

 of the Selenariida , Busk : here they are of enormous size 

 and of great strength, and assume, in some species at 

 least, a locomotive function, acting probably as oars, 

 and propelling the colony, which is free in the adult 

 state. In the history of these appendages we have a 

 curious illustration of the variety of function that may 

 connect itself with the same morphological element f. 



* It was first observed by Mr. Darwin in a species of Caber ea (' Voyages 

 of Adventure and Beagle,* vol. iii. pp. 259-262 ; ' Origin of Species ' (later 

 editions). I have also studied it in Caberea Boryi. 



In the passage referred to Mr. Darwin also states that the avicularia on 

 one side of a branch were observed to move, "sometimes coinstantaneously, 

 sometimes in regular order, one after the other." 



t Salensky finds in the vibraculum a structure homologous with the 

 polypide, corresponding exactly with that in the avicularium. 



The avicularia were first noticed by Ellis ; they have been investigated by 

 Nordmann (' Fauna Pontica'), Krobn, Vau Beneden (" Recherches," Mem. 

 Ac. R. Belg. vol. xviii.), Reid (Ann. & Mag. N. H. vol. xvi. p. 385), Darwin 

 {loc. cit.), Busk ("On Notamia:' Trans. Micr. Soc. Oct. 27, 1847; "On Avi- 

 cularia and Vibracula,"" vol. ii. p. 26), Smitt (Ofvers. Vet.-Ak. Forh. 1867, 

 p. 468), Nitsehe (Zeitsch. xxi. Heft 4, p. 110). 



^2 



