PHYSIOLOGICAL 229 



water, is seen in larval dragon-flies; and it is interesting to notice 

 that the violent expulsion of water from the posterior end of the 

 food-canal is just an exaggeration of the gentle respiratory currents 

 that are characteristic of these aquatic larvae. Somewhat unusual, 

 again, is the way in which lobsters and prawns jerk themselves tail- 

 foremost in the water by suddenly flexing the posterior body 

 (abdomen) forwards and downwards. This displaces a mass of water 

 towards the head. On occasion the common scallop (Pecteri), dis- 

 turbed by an enemy such as the starfish, can jerk itself off the 

 sea-floor with an energetic snap of its gaping shell-valves, and 

 continue swimming for some time by snapping the two valves 



Fig. 41. 



A Typical Octopus. A, one of the arms, with suckers (S). F, the funnel, 

 through which water is forcibly ejected from the mantle cavity (MC), 

 effecting locomotion. E, one of the eyes. 



together. Somewhat more intricate and much more beautiful is the 

 swimming of the nest-building Lima, a bivalve not uncommon in 

 the Firth of Clyde and in similar places. In some ways the strangest 

 mode of animal locomotion is exhibited by the common sea-urchin 

 (Echinus) on a firm, flat surface. This animal habitually moves by 

 means of its tube-feet, after the fashion of the starfish already 

 described; and it also utilises its spines, which are swayed on ball- 

 and-socket joints by basal muscles. But the Echinus is also able to 

 tumble along on the tips of the five teeth of its "Aristotle's lantern", 

 which project out of the mouth. The lantern can be swayed from 

 side to side by powerful muscles, and the locomotion may be de- 

 scribed, almost incredibly, as tumbling along on the tips of the 

 teeth. The track shows at short intervals the indentations of 



