4 Veterinary Medicine. 



account the relative injury caused by the individual parasite, and 

 the numbers by which the host is likely to be assailed. Some, 

 like trichina, echinococous, taenia fimbriata, or uncinaria, are so 

 deadly and so likely to undergo a constant increase in the same 

 locality in future years that their presence can only be looked on 

 as a growing menace to be abated at any cost or trouble. Some 

 are less gravely injurious, or increase less rapidly, so that they 

 are usually looked upon with little apprehension. There are 

 besides the commensals, like the analgesinse, or acari of the 

 feathers of birds, which are with good reason looked on as com- 

 paratively harmless. 



With the object of placing in relief the more prevalent and 

 habitually dangerous of the parasites, those that have been found 

 to be specially injurious or destructive have been printed in 

 blackfaced letters so that they can be readily picked out in a 

 hurried glance over the list. But too much importance must not 

 be attached to this conventional distinction, — the mere expression 

 of past experience, — since any injurious parasite will tend to in- 

 crease to a deadly prevalence when present in a given territory, 

 in which the numbers of its natural hosts are very great, and in 

 which other conditions conduce to its preservation and increase. 

 If it infests two different genera of hosts in its two successive 

 stages of larva and mature parasite, the presence of both genera 

 in large numbers is essential. If it must pa-ss a given stage 

 (embryo, larva) in water or in some invertebrate, then wet lands, 

 marshes, pools, lakes or sluggish streams are a necessary condi- 

 tion. If salt is destructive to embryo or larva, as in the case of 

 the trematodes then such waters must be fresh. If the larva, as 

 in the case of taenia canina, lives in an invertebrate skin-parasite 

 of the same host, then the existence and maintenance of the in- 

 testinal or other internal parasitism is dependent on the presence 

 of the cutaneous parasitism. If the parasite, Uke echinorhynchus 

 mu.st pass through its early immature stage in the larva of an 

 invertebrate like a maybug or cockchafer, then an outdoor life, 

 where the pig can grub-up and devour the invertebrate larva, is 

 the condition of becoming infested. Again, if the parasite, like 

 trichina, is usually taken in by devouring the smaller rodents 

 (rats, mice), or the food or water which they have contaminated, 

 then the excess in pigpens of such vermin, which have become 



