PHYSIOLOGICAL 227 



in the meantime another set of tube-feet have secured attachment 

 higher up. 



(II) The man may stand up in the boat and use a pole as a lever, 

 pressing it backwards against the floor of the stream. By this 

 "punting" he forces the boat forwards. This is one of the commonest 

 modes of animal locomotion, being exhibited by all the diverse types 

 that have firm appendages usable as levers against a firm base. A 

 beetle hurrying across the roadway, a crab walking on the rock of 

 the shore-pool, a frog jumping among the grass, an ostrich sprinting 

 at full speed, a horse at a gallop, a man walking or running — all are 

 using levers which propel the body forwards by pressing against a 

 hard substratum. Sometimes there are complications, which do 

 not essentially affect the principle of the method employed. Thus 

 the freshwater mussel may make its flabby "foot" (a muscular 

 protrusion of the ventral surface) tense with blood, close a sphincter 

 muscle which prevents back-flow, and then pull the ploughshare- 

 like organ backwards against the sand, thus pushing its body 

 forwards. The foot has to be protracted by other muscles before the 

 next step is taken. This method approaches pulling. Much in the 

 same way the cockle takes little leaps on the firm sand. 



The movements of snakes are somewhat intricate. A rapid dart 

 forward may be eflected by a sudden straightening of one or more 

 of the bays of the sinuous body, but let us take the ordinary smoothly 

 continuous progression, so remarkable in a limbless animal. Except in 

 burrowing snakes, the ventral surface is covered by a single series of 

 large scales, which can be raised and lowered. The posterior margins 

 of these scales are sharp, strong, and imbricating. When they are 

 raised, which is effected by special muscles, they tend to catch on 

 the roughnesses on the ground. A snake cannot move on a perfectly 

 smooth surface of glass or ice. Into the sides of the large ventral 

 scales the lower ends of the ribs are attached by minute ligaments, 

 and the upper ends are connected to the vertebrae by articulations 

 which allow them to be very readily moved forwards and backwards- 

 Several ribs are drawn forwards or headwards by muscles, thus 

 moving the associated scales a minute distance headwards. A whole 

 series of ribs and scales may be seen and felt working in the same 

 direction at the same time. Then these same ribs are drawn back- 

 wards, with the result that the pressure of the raised scales against 

 the hard ground pushes the body forwards. While one series of ribs 

 is being drawn backwards, another series is being drawn forwards, 

 and thus a continuous flowing movement is brought about. This 

 case is, perhaps, intermediate between "punting" and "rowing". 



(Ill) The man in the boat may stand in the stem and "scull", 

 using a single oar to displace masses of water alternately to right 

 and left. This is a common method among certain kinds of swimming 

 animals, such as fishes and whales. In most fishes the swimming 



