ECOLOGICAL 



89 



long ovipositor, inserts her eggs into the outer gill-plate of the mussel, 

 where they undergo a prolonged development, eventually giving 

 rise to larvae which make their way out of their temporary host. 



Several kinds of small fishes devour the aquatic larvae of mos- 

 quitoes, and thus check the spread of malaria. The Califomian 

 Gambusia is thus being introduced along the French Mediterranean 

 lagoon region. The freshwater snail LymncBus truncatulus is the 

 British host of the larval stages of the liver-fluke which causes "rot" 

 in sheep. The burbot is one of the hosts of the larval stage of the 

 Broad Tapeworm {Bothriocephalus latus), one of the most trouble- 

 some parasites of man. 



Utilisations of such increasing knowledge of the life-histories of 



Fig. 23. 



Cave Insect, with reduced eyes, but enlarged antennae (4), and 

 mouth-parts (i and 2). 



inimical species are thus now in active progress. (See section on 

 Biology in Medicine.) 



Minor Faunas. — {a) Of brackish water. — We are warranted in 

 speaking of a brackish-water fauna, because of its uniformity in 

 widely separated regions. It does not seem to be a mere physiological 

 assemblage, varying in each locality, but rather a transition fauna 

 of ancient date, a relic of a littoral fauna once more uniform. The 

 fact is that the power to live in brackish water is not very common ; 

 it runs in families. 



{b) Cave fauna. — In America, thanks very largely to the labours 

 of Packard, about 100 cave animals are known; in Europe the 

 number is about 300, the increase being largely due to the occur- 

 rence of about 100 species of two genera of beetles in European 



