204 
Mr. Home's Lecture 
structure, and actions, of the animals called hydatids, which ap- 
pear from their simplicity to he the furthest removed from the 
human ; for as the human is the most complicated, and most 
perfect in the creation, the hydatid is one of the most simple, 
and composed of the fewest parts. It is to appearance a mem- 
branous bag, the coats of which are so thin as to be semitrans- 
parent, and* to have no visible muscular structure. From the 
effects produced by the different parts of this bag while the 
animal is alive, being exactly similar to the contractions and 
relaxations of the muscular fibres in the human body, we 
must conclude that this membrane is possessed of a similar 
power ; and consequently, has the same right to be called 
muscular. 
The hydatid, from Its apparent want of muscles, and other 
parts which generally constitute an animal, was for a long 
while denied its place in the animal world, and considered as 
the production of disease ; we are, however, at present in pos- 
session of a sufficient number of facts, to ascertain, not only 
that it is an animal, but that it belongs to a genus of which 
there are several different species.. 
Hydatids are found to exist in the bodies of many quadru- 
peds, and often in the human ; the particular parts most fa- 
vourable to their support appear to be the liver, kidneys, and 
brain, although they are sometimes detected in other situations. 
One species is globular in its form, the outer surface of the 
bag smooth, uniform, and without any external opening ; they 
are seldom found single, and are contained in a cyst, or thick 
membranous covering, in which they appear to lie quite loose ; 
having no visible attachment to any part of it. This species 
is most frequently found in the liver and kidneys, both of the 
