PARASITIC DISEASES OF FARM ANIMALS. 



[From the "Field" 1890, by permission.'] 



ROUND WORMS (Nemathelminths) have a history which, as was staled 

 in the last article, is less clearly defined than that of the cystic or 

 trematode worms. 



In all the round worms the sexes are separate individuals, and the females 

 possess a long coiled uterus, in which are contained an immense number of 

 eggs, which, when they are extracted from the oviduct, have the living embryo 

 within the yet unbroken shell. 



When the embryo emerges from the egg, its organism is extremely simple. 

 A miniature representation of the parent worm in outline, its interior is filled 

 with granular matter, from which the digestive and sexual organs will be 

 formed ; and there is a narrow mouth, by means of which the young parasite 

 imbibes the nutritive fluid from the tissues in which it exists. 



There is nothing in the organization of the embryo to suggest the idea of 

 alternate generation, and all that is wanted to render the worm mature is the 

 development of the internal organs, which might be expected to appear in 

 due course. But after close observation, it is certain that the embryos do not 

 become mature in the organs of the animal in which the parent worms are 

 accommodated. And it is also a fact that myriads of eggs and embryos are 

 expelled by the infested animals, and fall, on the feeding grounds, where, 

 under favourable circumstances, their internal organs undergo development, 

 and it is at least probable that they become sufficiently mature by the follow- 

 ing season to play the same part in the organism of the higher animal that was 

 played by their parents before them. Experiments -which have for some time 

 been carried on at the Royal Veterinary College for the Royal Agricultural 

 Society have already led to discovery of important facts in the life-history of 

 thread worms, and have corroborated others which had previously been stated 

 or assumed by helminthologists. 



Dr. Spencer Cobbold, a short time before his death, observed that some of 

 the embryos of the Strongylus micrurus which he had under observation found 

 a resting place in a small earth worm which was by accident in the moist earth 

 in which the embryos were kept ; and in the report which Dr. Cobbold made 

 to the Royal Agricultural Society the fact was referred to, and its significance 

 was commented on. Further investigations, which are still being conducted, 

 have placed beyond doubt the fact that earthworms on lands where the lung 

 worm disease occurs are always infested with embryos ; and many of the young 

 worms attain a degree of development which approaches maturity during thei r 

 residence in the digestive canal of the earthworms. It is not improbable that 

 other creatures become temporary hosts to the embryos of thread worms, and 

 it is certain that they undergo development in moist earth. 



It was held at one time that infested pastures might be rendered healthy by 

 a winter's frost, or by a scorching sun ; and, possibly, some parasites may be 

 destroyed by cold or heat, but it cannot any longer be said that the lung worm 

 can be got rid of by such means. In the experiments which have been 

 recently recorded, embryos have been repeatedly frozen and thawed without 

 injury, and they have also survived the action of the sun, becoming lively when 

 moistened with water. It is, therefore, useless to expect much from climatic 



