526 PHILOSOPHICAL TRANSACTIONS. []aNNO IJQ^. 



whole chain of animated beings ; and from a review of the different circum- 

 stances in which it occurs, and the varied structure of parts on which it is im- 

 pressed, we shall have sufficient evidence that the fasciculated fibrous structure 

 commonly met with is not necessary to its existence, but only made use of for 

 its support and continuance. 



The structure which produces muscular action varies so much in different 

 animals, that we are at a loss to conceive how the effects should have the least 

 similarity ; and it is in some cases only from witnessing the actions, that we can 

 consider the parts as muscles ; since in nothing else do they bear a resemblance 

 to the muscular structure in the more perfect animals with which we are best 

 acquainted. We shall illustrate this observation by a description of the struc- 

 ture and actions, of the animals called hydatids, which appear from their simpli- 

 city to be 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 membranous 

 bag, the coats of which are so thin as to be semi-transparent, and to have no vi- 

 sible 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 possession 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 quadrupeds, and often in 

 the human ; the particular parts most favourable to their support appear to be 

 the liver, kidneys, and brain, though they are sometimes detected in other situ- 

 ations. One species is globular in its tform, 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 quadruped and hu- 

 man subject. They vary in size,; but those most commonly met with, are from 

 i of an inch to f of an inch in diameter. 



Another species is of an oval form, with a long process, or neck, continued 

 from the smallest end of the oval, at the termination of which, by the assisUmce 

 of magnifying glasses, is to be seen a k'ind of mouth ; but whether this is in- 

 tended merely for the purpose of attachment, or to receive nourishment, is not 



