THE MOVEMENTS OF SHORE ANIMALS 163 



comparable to the mode of progression of a caterpillar, save 

 that only the creeping surface and not the whole body partici- 

 pates in the action. The lateral margins frequently do not 

 share in this motion, but have a gentle, lateral, undulating 

 movement of their own." The foregoing description is by 

 no means applicable to the gliding movements of all Gastero- 

 pods. In Littorina, for instance, the foot is divided into 

 two halves by a median groove, and the animal moves by 

 lifting and advancing the alternate halves, the mode of 

 locomotion being comparable, according to Woodward, with 

 that of a man with his ankles tied together. Robert (1908), 

 in discussing locomotion in Haliotts, Helix, Chiton, Littorina, 

 and other Gasteropods, maintains that two modes of muscular 

 undulation in the foot — from in front backwards and vice 

 versa — may cause progression, but the waves are not always 

 equally distinct, and the matter does not seem to admit of 

 short formulation. Vies (1907) divides the different types 

 of wave-motion to be seen on the foot of creeping molluscs 

 into two groups : (i) direct, i.e. forwards in the direction of 

 progress ; and (2) retrograde, i.e. from front to back. The 

 group of " direct " forms is further subdivided as follows : 



{a) monotaxic : one or more waves visible traversing 

 whole width of foot, e.g. Helix, Limax, Arion 

 (terrestrial) ; 



{b) ditaxic : two systems of waves, each occupying half the 

 foot, with the median line unaffected, e.g. Haliotis, 

 Trochus ; these are rapidly moving forms ; 



(c) tetraxic : four systems of waves, two sets of lateral 

 alternating waves ; seen in small species oi Littorina. 

 The group of " retrograde " forms contains the following 

 types: 



(fl) monotaxic, e.g. Chiton ; and 



{b) ditaxic, e.g. Littorina littorea and L. riidis. 



Swimming. — In addition to creeping or gliding over the 

 substratum (stereotropic) numerous shore forms, leaving 

 fishes on one side, possess in more or less developed fashion 

 the power of swimming (pleotropic). It is, in fact, a feature 



