300 



FILARl^E AND THEIR ALLIES 



worms reach the blood by way of the lymph stream and these 

 grow to about 300 /* (a little over T ^ of an inch) in length. They 

 are delicate colorless worms (Fig. 124 A), blunt at the anterior 

 end and tapering to a slender point at the tail end, and are 

 entirely enclosed in a remarkably delicate transparent sheath, 

 which, although it fits as tightly as a glove over a finger, is too 

 long for the animal and can be seen projecting at either end. The 

 sheath may be looked upon as a wonderful adaptation to prevent 



the worms from being able to 

 bore through the bloodvessels 

 and escape from the blood, in 

 which case they would miss 

 their chance for " salvation." 

 The internal organs are in a 

 very rudimentary condition. 

 The most remarkable cir- 

 cumstance connected with the 

 life of these microfilariae is the 

 periodical appearance and dis- 

 appearance of them in the 

 blood of the peripheral vessels. 

 If the blood of an infected 

 person is examined during the 

 day few if any worms can be 

 found, but as evening ap- 

 proaches they begin to appear 

 FIG. 124. Comparison of microfiiariae ; and continue to increase until 



A, mf. bancrofti (large with sheath); B, fl u nil f mirlnip-ht flftpr whinh 

 mf. perstans (small, blunt tail, no sheath); aD mgllt > alt< 



c, mf. loa (large, with sheath); D, mf. they decrease again until 



i n morni "g- Durfn s the ^ ht 



when they are most abun- 

 dant there may be as many as 500 worms in a single drop of 

 blood. If the parasites are assumed to be evenly distributed 

 throughout the peripheral circulation, this would imply the 

 presence of several million worms in the body. The periodic 

 appearance and disappearance of microfilarise in the blood is not 

 invariable. When an infected person is made to sleep in the 

 daytime instead of at night, the appearance and disappearance 

 of the parasites in the peripheral bloodvessels can be reversed, 

 implying that the distribution of the parasites may be dependent 



