122 PRACTICAL PARASITOLOGY 



The Trichocephales (T. affinis of sheep, T. crenatus of swine, 

 T. depressiusculus of dogs, T. unguiculatus of rabbits and hares, 

 T. nodosus of rats) supply material for direct infection by means of 

 eggs containing embryos. The eggs should be removed from the 

 uterus and cultivated in water or damp earth. As a rule, development 

 proceeds slowly and many eggs perish in the process. As soon as the 

 embryos are formed, the eggs are mixed with a suitable vehicle, such 

 as milk, sop, &c., and introduced by the mouth into the animal 

 selected for the host. According to Leuckart and Grassi, Tricho- 

 cephales will be fully formed at the end of four to five weeks and may 

 be seen microscopically in the caecum. The young forms, which are 

 exceedingly small and only to be detected with the aid of the micro- 

 scope, will be found in the intestinal mucus. They appear six to ten 

 days after infection never later, though sometimes earlier and they 

 will be found in an already infected animal, if a second infection is 

 induced a few days before examination. 



The Aiikylostoma varieties may be taken as examples of parasites, 

 the larval stage of which emerges from the egg and attains a certain 

 stage of development before introduction into the final host. A good 

 substitute for the A. duodenale of man is A. trigonocephalum, found 

 in dogs and foxes. The dog should be used for experiment. The 

 eggs are best cultivated in the faeces of their host by the method 

 described above, bone-black being mixed with the faecal material and 

 water used to dilute it. The larvae are isolated by pouring off the 

 water and filtering it. They invariably fall to the bottom of the 

 water and will remain alive in it for months. They are useful for 

 purposes of infection only after they have shed one skin and are 

 preparing to shed another. This takes place soon after they have 

 left the shell. The ripe larva is recognized by the presence of an 

 envelope, detached at the head and tail, which represents the cuticle 

 immediately before sloughing. Certain changes will also have taken 

 place in the shape of the body and the pharynx. This description 

 applies equally to larvae of A. duodenale of man. According to 

 Leuckart, the sloughing of the larvae of A. trigonocephalum of dogs 

 takes place during the free stage. This parasite, like the species 

 found in man, does not undergo any further transformation until 

 after its introduction into the body of the definite host. This is 

 effected directly, by means of water or earth containing larva?. If 

 infection is very severe, the host will perish in about ten days. 

 Vomiting sometimes occurs soon after infection, and this is very 

 liable to endanger the result of the experiment. The best hosts are, 

 undoubtedly, young dogs. 



The difference in structure, especially in that of the excretory 

 organs, between the larva and the adult parasite is very great and is 



