V . DRACUNCULUS, FILARIA 205 



into it a little corrosive sublimate solution. In the latter case the dead 

 remains of the worm are gradually destroyed by the living activities of 

 the surrounding tissue. 



FiLARIA BANCROFTI 



This is a slender thread-like worm, the female about 100 mm. in length 

 and the male about 40 mm. It occurs as a parasite of man in widely 

 distributed localities throughout the Tropics and inhabits the lymphatic 

 spaces. The young when born pass into the lymph and have a very 

 characteristic appearance. They are about -3 mm. in length and each 

 is enclosed in a loosely-fitting tubular membranous sheath closed at each 

 end in which it lashes actively backwards and forwards. The sheath 

 is simply the egg-shell which has entirely lost its rigid character and 

 become quite soft. The young worms pass from the lymph into the 

 blood in which there may be several millions present without producing 

 any apparent ill-effects on the health of the host. 



When the blood of an infected person is examined microscopically 

 in the ordinary way — by taking a drop from the skin — the worms are 

 found to present an extraordinary periodicity. During the day they 

 may be completely absent. Towards sundown they begin to make their 

 appearance and during the evening their numbers undergo a rapid in- 

 crease, until between 10 and 12 o'clock the blood is swarming with them. 

 Then they gradually decrease again in numbers until by 7 or 8 a.m. they 

 have almost entirely disappeared. 



This periodic appearance and disappearance of the worms was a 

 great puzzle until Manson established the fact that during the hours of 

 daylight they do not pass out of existence but merely frequent the 

 deep vessels of the body, congregating in the great arteries and more 

 especially in the lungs, in which organ they collect in the capillaries as 

 well as in the larger vessels. The migration from these deeper vessels 

 into the vessels of the skin turns out to be an adaptive arrangement, 

 correlated with the fact that part of the life-history is passed in the 

 body of night-flying mosquitos of various species. 



When blood containing the young worms is taken in by a mosquito, 

 the worms continue their active movements • within the sheath. The 

 sheath is now restrained in its movements by the viscid contents of the 

 mosquito's alimentary canal and after a while the worm, butting against 

 now one and now the other end of the sheath, breaks its way out of it. 

 It soon (within six to twelve hours) bores through the wall of the ali- 

 mentary canal and takes up its position amongst the muscles of the 



