598 Abstract Report of Agricultural Discussions. 
like justice to it in a single lecture, or, indeed, in a course of twenty 
lectures. The difficulty therefore is to epitomise the matter. Under 
these circumstances, he proposed to select one or two prominent affec- 
tions to which domesticated animals are subject, and explain some- 
what of the natm-al history of the parasites which are their superin- 
ducing cause. Parasites ai-e met with in all structiu-es of the body 
— not only in the internal organs, but also upon the external parts of 
the frame ; and whether in the one situation or the other, they are 
more or less productive of mischief, provided they exist in sufficient 
numbers. It is not an imcommon observation that a few worms can do 
no great harm ; there are, however, affections very destructive of the 
health and the lives of animals, which really depend upon the existence 
of worms, and often not in large numbers. 
Parasites are thus classified : — First, the Epizoa, which live on the 
skin of animals ; secondly, Entozoa, which inhabit the internal struc- 
tures, without reference to location in any particlar organ ; and 
thirdly, Ectozoa, which occupy either the internal or external portions 
of the body for a certain time, whilst undergoing some of the transmu- 
tations through which they pass. 
As familiar examples of the first of these classes, he would point to 
lice, ticks, mites, and so forth, well known as having their habitat on 
the skins of animals, and being productive of mischief in proportion 
to the numbers in which they exist. The acarus ovis, met with upon 
the skin of the sheep, is the direct cause of the affection called scab, 
and also of mange in the horse ; in fact, the true mange of the horse 
and the scab of sheep are identical, and depend upon the presence of 
this parasite. 
Of the ectozoa, the most familiar example is the bot in horses. 
Here a fly {cesirus equi) deposits its ova on the hair, which, by the 
licking of the horse's tongue, are carried into the mouth, and conveyed 
with the food into the stomach. The ova are quickly developed into 
larvEe or grubs M'hen exposed to the heat of the mouth and the saliva. 
When they reach the stomach they attach themselves to its mem- 
branous lining by their booklets, and there feed, not upon the stomach 
itself, but upon its contents, from the latter part of the summer, thi'ough 
the autumn and winter, imtil about midsummer in the following year, 
when they attain matm-ity. They then lose their power of hanging on, 
di'op from their stations, mingle with the contents of the stomach, ^jind, 
l>assing through the alimentary canal, are expelled fi'om the body. 
Upon expulsion the chrysalis is fonned from the larva, and in a few 
days the fly bm'sts forth. These creatui-es therefore exist for a con- 
siderable time altogether independent of their host. 
The Entozoa. 
The class, truly teimed entozoa, inhabit the internal structures of 
the body, dwelling, some of them aj)parcntly their entire lives, in that 
situation, and multiplying in the organ in which they are found. 
3Iany so-called jiarasitic-worms exist as ova, and, in other Ibrms, in 
jjools and iu sewage, and being accidentally conveyed into the bodies 
