ANIMAL DISEASES. I45 



and that in general the species of a genus of worms possess 

 common egg characters. Thus it will be possible by the examina- 

 tion of eggs found in the faeces, to determine the nature of the worm 

 infection. Already a considerable number of egg photographs 

 have been secured, and the work is approaching completion. 



A further " general " line of investigation refers to the method 

 by which animals are infected with worm parasites. It is known 

 that certain round-worms which attack man, such as the hook- 

 worm, are able to enter through the skin. The larva throws 

 off its sheath, and penetrates through the skin into the blood 

 stream, whence it finds its way via the lungs into the intestines. 

 It is important to know whether this power is possessed by any 

 of the worms affecting farm stock, for if so the latter could contract 

 disease merely through lying on infected pastures. An ingenious 

 method has been devised for ascertaining whether any species 

 of worm has the power of boring through the skin, and this method 

 is being used in an investigation of the worm parasites of horses 

 and sheep. 



A series of experiments has also been commenced to find out 

 whether it is possible for damage to be caused to an animal by 

 the larvae of parasitic worms which are not normally parasitic in 

 it. For example, it is known from recent work in this country 

 and the United States that the eggs of the round-worm of man 

 and the pig, A scans hinihricoides, when administered to a number 

 of other animals, mice, rats, guinea pigs, rabbits, and lambs, set 

 free their larvae on reaching the intestine. These larvae bore 

 their way through the intestinal wall, and, getting into the blood 

 stream, travel to the liver and lungs, where they produce lesions 

 and finally reach the intestine again by way of the wind-pipe and 

 oesophagus. Such larvae do not attain maturity in the intestine 

 of the new host, and in most cases are discharged from the body. 

 Near relatives of the round-worm of the pig are known to occur 

 in certain grazing animals, e.g., the horse and the ox, and their 

 eggs on passing out with the droppings, must be widely distri- 

 buted over pastures where the infected animals feed. These 

 eggs may easily be picked up by other grazing animals put on to 

 the infected pastures. It is therefore of considerable importance 

 to find out whether such eggs can become infective to other animals 

 taking them in during their cropping of the herbage, and whether 

 their larval stage can be set free in the intestine and subsequently 

 cause lesions in the liver and lungs of the new host. It has 

 usually been assumed that such larval stages, belonging to para- 

 sites not normal to a given host, would be destroj^ed by the 

 digestive juices of the animal, but there is reason to suppose 

 that a parasite which is not found in the adult stage in the 



