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INTRODUCTION TO ZOOLOGY 



said to be formed of two fused appendages, and bearing con- 

 centric rows of hooks (Fig. 280, A) which, together with the 



three sucker - like 

 projections from the 

 end of the body, 

 enable the larva to 

 keep firmly fixed 

 even in a strong 

 current. On the 

 first thoracic seg- 

 ment also, there is 

 a single finger-like 

 process beset ter- 

 minally with hooks, 

 formed from two 

 fused appendages. 

 By means of these 

 appendages at the 

 two ends of the 

 body, the larva 

 creeps about fairly 

 actively, moving in 

 a leech - like way, 

 fixing the front of 

 the body and then 

 drawing up the hind 

 end and fixing that, 

 before throwing the front end forward again. The head bears 

 the most remarkable organs of the larva, the two plume-like 

 structures, each formed of many long filaments which are 

 constantly in motion driving food into the mouth. These 

 plumes are immediately folded together and withdrawn if the 

 larva is startled, and only gradually unfolded again. If picked 

 up and dropped into the water, the larva falls, but in so doing 

 spins a thread so fine as to be almost invisible. Having reached 

 the bottom, it climbs up again by this thread, holding it 

 between the thoracic appendage and the mouth. Professor 

 Miall has found that in their native streams, if alarmed in 

 any way, these larvae at once drop on such a thread from their 

 support, and climb up it again to their original position when 

 the danger is past. Sometimes a whole network of such threads 



Pig. 280. — The Larvae of SimuHwm. 

 A, Ventral view ; B, dorsal view ; a, fused anterior 



