BEARING HELMINTHES FEEDING EXPERIMENTS 117 



which is used to preserve them, and are frequently undamaged by 

 drying. 1 



For cultures in faeces, especially of the eggs of Ankylostoma 

 duodenale, Looss 2 recommends the admixture of an equal quantity 

 of bone-black. This not only removes all odour, but it also favours 

 the development of the worms. Water should not be added, but 

 the culture should be frequently well stirred in order to bring the eggs 

 which are at the bottom into contact with the air. 



The manner in which the embryo leaves the shell, or other 

 covering, varies considerably. Some varieties, whether formed before 

 or after the deposit of the egg, cannot be induced to emerge in the 

 open. Such are those of the Acanthocephales, the majority of the 

 Cestodes, and many Trematodes and Nematodes (Ascaris). The 

 embryos of certain other varieties, such as those of the 

 liver-fluke (fig. 44), and the broad tapeworm, emerge 

 readily and move about in the water or earth (Nema- 

 todes) of the culture. In the case of embryos which 

 emerge in the open, their free existence may last for 

 a comparatively long time. The young worms feed, 

 grow, cast their skins and, in the case of one group 

 (Angiostomides), become sexually mature. This in- 

 stance of heterogony is extremely interesting, and the 

 process may be watched by means of experiments in 

 the following manner. 



A r>7 ,j FIG. 44.-Mira- 



A small roundworm, Rhabdonema nigrovenosnm, cidium of the 



belonging to the Angiostomides, is found in the lung of liver-fluke im- 

 mediately after 

 the brown frog. As soon as the eggs are deposited, they leaving the egg. 



make their way through the branches of the lung to the Magnified. (After 



Leuckart.) 



oral cavity. They are then swallowed by the frog and 

 thus find an entrance into the alimentary canal. As they traverse it, the 

 development of the embryo becomes complete, and the young worms 

 usually emerge from the shell which is oval in shape and very thin 

 in the terminal portion of the large bow r el. The number of eggs 

 deposited is so large that, if Rhabdonemce&Ye present in the lungs, eggs 

 containing embryos, or young worms which have left the cell, are 

 certainly present in the lower bowel. These young worms are about 

 0'4 mm. long and are furnished with a double resophageal swelling. 

 They may be brought to sexual maturity in the following manner. 



1 There is no fixed line of demarcation between water and earth cultures. The 

 eggs of Trichocephales, for instance, develop in water as well as in mould ; those of 

 the liver-fluke will develop in mould if it is wet enough ; and those of Ascarides do 

 equally well in either medium. 



2 A. C. Looss, Centralb. f. Bcukt., Paras, und Inf., xx, 1886, p. 866. 



