44 FREE AND FIXED ANIMALS 



remarkable and suggestive fact that the typically 

 pelagic Alciopidae, freed entirely from their stereo- 

 tropic bonds, are usually very poor swimmers. 



The case of Nephthys is worthy of special 

 mention. This is one of the best swimmers 

 known amongst Annelids, although when left 

 alone it passes most of its existence buried in 

 the sand. Eisig found that, unlike Psammolyce 

 which paddles along the bottom but never 

 swims, Nephthys on the contrary undulates 

 rapidly through the water, but never creeps along 

 the bottom ; it is either at rest or swimming, 

 passing abruptly from the one condition to the 

 other without an intervening reptant phase. 



An analogous instance of spasmodic pleo- 

 tropism on the part of a normally stereotropic 

 and partly statozoic organism is the case of 

 the bivalve mollusc, Pecten the scallop. " If 

 disturbed the attached scallop can break or 

 cast off its byssal threads and swim by clapping 

 the shell " ; the gaping ventral lips of the shell 

 being directed forwards in locomotion. 1 



The fact, elucidated by Dr Eisig, that the 

 posture of the Annelid appendages or podia on 

 opposite sides of the arcs is the same when the 

 worm is progressing slowly along the bottom 

 as when swimming rapidly through the water, 

 has led him to the conclusion that the stereo- 

 tropic movement (on the bottom) is secondary 



1 Cf. W. J. Dakin, "Memoir on Pecten," L.M.B.C. Memoir 

 XVII., Liverpool, 1909. 



