98 THE STRUCTURE AND ECONOMY 



They are also very numerous in the larger branches of the 

 nerves, and particularly on those of the pectoral and caudal fins. 

 In the former situation they are suspended in the gelatinous 

 fluid by fibres of areolar texture and by blood-vessels ; in the 

 latter they lie embedded in the substance of the nerve, the ulti- 

 mate fibres of which are spread in bundles over the surface of 

 the cysts. 



The cysts are produced spheroids, somewhat flattened ; their 

 long axis measures about one-fourth of a line. 



They consist of three tunics ; an external, which appears to be 

 derived from the areolar texture of the infested animal, and of a 

 middle and internal, belonging to the parasite. 



Upon the surface, and in the substance of the external tunic, 

 the blood-vessels of the nerve can occasionally be seen, and re- 

 cognised by their contents. One or two vessels may thus be 

 observed coasting along the cyst, accompanied by single nerve 

 tubes, or by bundles of these, or by a mass which completely 

 encloses and conceals the cyst. The second tunic is a fine trans- 

 parent membrane, which lines the first, and has in its turn its 

 internal surface covered by an epithelial layer, which is the third 

 tunic of the cyst. The epithelia are flat, irregular in shape, and 

 somewhat opaque. The third, or internal layer, formed by them, 

 breaks up under the pressure of the glass plates, so as to present 

 rents or fissures passing in various directions over it. 



The cyst, in addition to the worm, contains a small quantity 

 of fluid, in which oil-like globules of various sizes float. 



The worm is a Distoma, oblong, dilated in front, tapering 

 slightly towards its posterior extremity. The mouth longitudi- 

 nally oval, and rather pointed posteriorly, is surrounded by the 

 usual suctorial disc. The acctabulum is situated at the junction 

 of the anterior and middle third of the animal, and can be pro- 

 truded from the surface of the body. 



On the anterior edge of the acctabulum a minute pore is si- 

 tuated, and communicates with a sac, to be afterwards described. 



At the posterior extremity of the animal another orifice is 

 placed, which forms the outlet of the large chyle sac, and ap- 

 parently also of another sac, to be afterwards alluded to. 



The integument of the two anterior thirds of the body, is 



