494 ANATOMICAL AND PATHOLOGICAL OBSERVATIONS. 



few exceptions almost every specimen is converted into the 

 hard cretaceous matter spoken of ; many, at the same time, 

 presenting all the intermediate stages of decay. Gymno- 

 rhynchus presents us with a very curious habit dependant 

 upon this mode of structure, and which enables the animal to 

 avoid the death from which all its co-geners suffer. This 

 species, which I have fortunately had an opportunity of ex- 

 amining in its natural habitat, but which has been already 

 described by my brother (Edinburgh Philosophical Journal, 

 vol. 31), inhabits the liver of the sun-fish in great numbers, 

 and from its peculiar structure is enabled to move slowly 

 through the organ it infests. If the cyst of this worm is 

 carefully examined, it will be found that the inner membrane, 

 containing the absorbent cells, is covered anteriorly with a very 

 thin layer only of the external membrane, so that it is enabled 

 to absorb the nourishment from the external textures in great 

 abundance, which thus enables the animal to move forward as 

 well as obtain a supply of food ; as we trace the cyst back- 

 wards, the external membrane will be found to become thicker 

 and thicker, as also more impermeable, until we reach the tail 

 of the animal, after which it becomes a mere cord. This cord 

 can be traced for a great distance, becoming less and less per- 

 ceptible, until it is lost altogether, and the course only marked 

 by a simple line of a darker colour than the rest of the textures. 

 It will be observed that the external membrane of this animal 

 presents analogies similar to that of Acephalocystis ; for in- 

 stance, the cephalic portion of the membrane is so thin as to 

 be hardly distinguishable, being thus analogous to the young 

 hydatid. 



In regard to the cyst of these worms, it has been long a 

 question how far it is a part of the enclosed animal. Professor 

 Owen* holds that it is merely condensed textures of the 



* Owen, "Description of a Microscopic Entozoon infesting the Muscles of 

 the Human Body." Transactions of the Zoological Society, vol. i. p. 322. 



