SWIMMING ANNELIDS 43 



It is sometimes customary to speak simply of 

 positive and negative reactions, but it should not 

 be forgotten that there are all degrees of stereo- 

 tropism, from an occasional or facultative mani- 

 festation to a chronic condition, leading on to a 

 definite sedentary habit. Many cases amongst 

 the Annelid worms could be instanced where 

 stereotropism and pleotropism (the free swimming 

 habit) exist side by side, the latter frequently 

 only manifesting itself at the breeding or swarm- 

 ing season, as with epigamous Nereids and the 

 celebrated Palolo worm (Eunicidse) of Samoa 

 and elsewhere in the South Seas. 



Professor Hugo Eisig 1 has made exact observa- 

 tions upon a number of Annelida Polychseta, re- 

 presenting ten families, some of which exhibit 

 a high degree of stereotropism together with an 

 equally high degree of pleotropism, e.g., Lepid- 

 asthenia and Ophiodromus. Others again like 

 Psammolyce and Diopatra, never advance beyond 

 what is described as a swimming gait, i.e., crawl- 

 ing along the bottom with a horizontal undulating 

 movement of the body producing alternate arcs 

 or waves of progression. When the amplitude of 

 these undulations surpasses a certain magnitude, 

 the rapidity of movement is thereby increased to 

 such an extent that the animal rises from the 

 bottom and swims through the water. It is a 



1 H. Eisig, Ichthyotomus sa?iguinarius. Monograph 28 in " Fauna 

 und Flora des Golfesvon Neapel," Berlin, 1906, see pp. 190-267. 



