118 



PRACTICAL PAEASITOLOGY 



The contents of the lower bowel of an infected frog are carefully 

 freed, with the aid of a magnifying glass, from other parasites (larger 

 Nematodes and Trematodes). The mass is then transferred in small 

 portions to a watch-glass, the surface of which has been previously 

 covered with damp earth. The glass is covered over and put into 

 a damp chamber. If the external temperature is high, the young 

 worms will slough their skin and attain sexual maturity in about a 

 day. In the winter, on the contrary, the process may occupy a week. 

 The body is extremely transparent and allows the internal organs to 



be easily examined. The smaller male, which 

 is recognized by its spicule (fig. 45), dies soon 

 after copulation, while the fertilized eggs 

 develop in the uterus of the female. The 

 young worms emerge from the shells, break 

 through the uterine wall, and grow up within 

 the body-cavity of the mother, whose in- 

 ternal organs they completely destroy. At the 

 end of four to five days, in summer, nothing is 

 left of the mother but the cuticle, in the in- 

 terior of which the worms make lively move- 

 ments. As soon as they leave the cuticle and 

 reach the open air, their rhabditis form, which 

 is characterized by two pharyngeal swellings, 

 becomes modified. After that they do not 

 change. They wait until an opportunity 

 offers, when they make their way into the 

 larynx of a frog and thence into the lungs. 

 This may be accomplished artificially, by 

 transferring a portion of the slime containing 

 the modified young worms to the mouth of a 

 frog. The swallowing of the mass is prevented 

 by holding the mouth open, while the entrance 

 to the lung which is situated upon the ventral 

 side at the base of the larynx, is held apart 

 with tweezers, in the expectation that some 

 of the worms will find their way into the 

 lungs. Though, of course, not invariably successful, this experiment 

 is attended by results in the majority of cases' If the slimy mass is 

 swallowed the worms will die in the- stomach of the frog. 1 



A similar developmental process, consisting in an alternation of 

 parasitic and free-living generations, is observed in the tropical form 



FIG. 45. Male of the free- 

 living generation of Rhab- 

 donema nigrovenosum. Mag- 

 nified. (After Leuckart.) A., 

 Anus. D., Small intestine. 

 M., Mouth. Oes., (Esophagus. 

 P., Papillae at the tail-end. 

 Sp., Spicula. 



1 K. Leuckart, "Die menschl. Parasiten," vol. ii, Leipzig, 1876, pp. 139-148. 



