Genera of the ITorth American Palaeozoic Bryozoa. 445 



into a seta, but the beak survives and the movement is prob- 

 ably nothing more than the rising and falling as of a lid. In 

 Mastigophora Ilyndmanni, the mouth is so modified as to give 

 much more play to the seta, which is thrown backward and for- 

 ward with perfect freedom, and has much the appearance of a 

 lash. In this species the vibraculum is borne on a distinct cell, 

 resembling the zooecium (on which it is developed), except in 

 size. In yet another case the vibracular cells are still further 

 reduced, and one is placed on each side of the orifice of the 

 zooecium. When we come to the higher forms, we meet with 

 cases in which the seta resumes the dimensions of the mandible, 

 and loses its free and vigorous swing. The most elaborate form 

 of this appendage is found in the genus Caberea. There the 

 chamber is large and traversed on the upper side by a channel 

 or groove, in which the seta lies when at rest. The latter is of 

 great length and serrated or toothed along 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 swinging to and fro at the same moment, and as if under 

 a common impulse. We can hardly doubt but there must be 

 some intercommunication between the nerve centers of the indi- 

 vidual vibracula, on which these combined movements depend, 

 but so far the synchronism has attracted very little attention, 

 and we have no observations that throw an}^ further light upon 

 it. The setae attain their highest development in the family of 

 the Selenariid^, 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 appen- 

 dages we have a curious illustration of the variety of function 

 that may connect itself with the same morphological element." 



The Animal. 



In the Gi'MNOL^vfATA the principal species studied have been 

 the marine form, Bowerhankia densa^ and the fresh-water form 

 Paludicella Ehrenbergi. These two forms, though differing some 

 in detail, are of essentially the same structure. 



For the Hippocrepian forms Alcyonella fungosa furnished the 

 principal material. 



