302 



FILAR liE AND THEIR ALLIES 



Once free, the little larvae (Fig. 125B) move actively about in 

 quite a different manner from the ineffective wriggling in which 

 they indulged while enclosed in the sheath, and by means of which 

 they were unable to " get anywhere." The active liberated 



worms make their way 

 to the thoracic muscles 

 of the mosquito, where 

 they He between the 

 nmscle fibers and par- 

 allel with them. The 

 body, growing rapidly, 

 by the fourth to tenth 

 day becomes thick and 

 sausage-like (Fig. 

 125C), with a short, 

 pointed tail, but it later 

 increases greatly in 

 length and decreases 

 slightly in thickness, 

 thus becoming long and 

 again (Fig. 

 125D). Meanwhile the 

 internal organization of the animal undergoes a great change. 

 The central core of cells gradually becomes differentiated into a 

 digestive tract, separated from the body wall by a true body 



Fig. 125. Development of Filaria hancrofti in 

 mosquito; .4, as withdrawn with tilood (first 24 hours) 

 in stomach; B, form found in tissues just outside 

 stomach (48 to 72 hours after ingestion) ; C, form 

 found in muscles on fourth day; D, mature larval 

 form, ready for transmission, in proboscis (two or 

 more weeks after ingestion). X 150. (After Lewis slender 

 from Nuttall.) 



Fro. 126. Mature larvaj of Filaria hancrofti in thoracic muscles and proboscis 

 of mosquito. (After Castellani and Chalmers.) 



cavity. By the time the larva has reached its full size — about 

 1.5 mm. (i^ff of an inch) in length — the digestive tract is a com- 

 plete tube with both mouth and anal openings. While these 

 changes are taking place, the larval worm, though capable of 

 activity, remains at rest between the nmscle fibers (Fig. 126), 



