OF THE CYSTIC ENTOZOA. 9;> 



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 examining in its natural habitat, but which 

 has been already described by my brother (Edinburgh Philoso- 

 phical 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 backwards, 

 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 perceptible, 

 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 instance, the ce- 

 phalic portion of the membrane is so thin as to be hardly distin- 

 guishable, being thus analogous to the young hydatid. 



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

 tion how far it is a part of the enclosed animal. Professor Owen* 

 holds, that it is merely condensed textures of the infested being, 

 and Dr. Knoxf again, that it belongs essentially to the parasite. 

 My brother, in the Paper already alluded to, says, regarding the 

 cyst " May we not suppose them to be parts of the original 

 ovum, within which the animal was formed, and within which it 

 passes its term of existence." From observations made on the 

 developement of the acephalocystic entozoa, it may be safely 

 stated, I think, that the above statement is correct, for acephalo- 

 cystis must be considered as an enlarged ovum ; but Sphairidion 

 perhaps is the best example of this peculiar mode of formation, 



* Owen. " Description of a Microscopic Entozoon infesting the Mtiscles of the Human 

 Body." Transactions of the Zoological Society, Vol. I., page 322. 

 t Knox, Edinburgh Medical and Surgical Journal. 



