MOVEMENTS OF AQUATIC ANIMALS 33 



nautilus inhale deep draughts of water under their 

 gill-covers and expel them through a funnel, strongly 

 when at rest, violently when they wish to make 

 a great backward leap. Many aquatic insect-larvae, 

 for instance those of dragon-flies, continually inhale 

 draughts of water, and merely increase the force with 

 which they expel the draught when they wish to make 

 a darting movement. Prawns and lobsters leap back- 

 wards by a somewhat different artifice, laying hold 

 of the water by the broad, concave under-surface of 

 the tail, and then bending this smartly beneath the 

 body. 



Another mode of motion is that adopted by star- 

 fish and sea-urchins. The five arms or five hoops 

 that surround the bodies of these creatures are provided 

 with serried ranks of hollow tentacles ending in suckers. 

 When the starfish wishes to move it advances one arm, 

 stretches out the suckers belonging to it, and draws the 

 body up behind. In the same way a sea-urchin emits 

 long slender tentacles from its shell, and, advancing 

 these, draws the body in tow. The brittle'stars which 

 have such tentacles but no suckers at their tips, 

 stretch out one arm in the direction they wish to 

 follow, and, using the others as fins, strike the ground 

 with a backward sweep, and shuffle over it in an un- 

 gainly fashion. 



The movements of animalcules : Ciliary move- 

 ment. The movement of most animalcules is due 

 to minute elastic hairs, which act as so many oars 

 projecting from the body. Unlike oars, however, these 



D 



