302 



FILARLE 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 lie between the 

 muscle 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 



FIG. 125. Development of Filaria bancrofti in increases greatly in 

 mosquito ; A , as withdrawn with blood (first 24 hours) 

 in stomach; B, form found in tissues just outside 

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



length and decreases 

 slightly in thickness, 



in 



found in muscles on fourth day; A mature larval thus becoming long and 

 lorm, ready ior transmission, in proboscis (two or . 



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



from Nuttaii.) 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. 126. 



Mature larvae of Filaria bancrofti 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. (^ 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 muscle fibers (Fig. 126), 



