WAGENER^ ON GYRODACTYLUS ELEGANS. 199 



the formation of which^ besides the integument, the visceral 

 fleshy substance of the body occasionally enters. Oil-drops 

 of greater or less size are scattered through the entire body, 

 and are very manifest in the perfectly fresh animal, which, 

 in that condition, may be said to be as clear as glass. The 

 opacity, which very soon comes on under the microscope, is 

 (Jbnnected, for the most part, with endosmotic conditions of 

 the external integument. 



The caudal disc presents a central and a peripheral 

 portion. It corresponds, in every particular, with the syno- 

 nymous organ of some species of Dactyloyyrus, with the one 

 exception that, in these case^, the points of the hooks are 

 directed towards the dorsal, whilst in Gyrodactylus they 

 look towards the ventral aspect of the body. 



The central part of the caudal disc is constituted by a 

 fleshy bundle, very finely striated longitudinally, which com- 

 pletely surrounds the large hooks with their two lateral 

 processes. The point of each hook corresponds to an open- 

 ing in this sort of cushion, and on the edge of this opening, 

 towards the body of the animal, a slender streak, curved into 

 the form of a V, forms a partial border to the orifice. 



The hook apparatus of the central part of the disc consists 

 of two large flat hooks, each with its two transverse lateral 

 appendages. They lie on the edge, and are curved upon it, 

 the base being enlarged towards the edge in an irregular way 

 difficult to describe. On the sides of the hooks, looking 

 towards each other, are two projecting sub- parallel folds or 

 elevations, corresponding to which are depressions on the ex- 

 ternal surface. These folds are short, of a circumflex form, 

 and run from behind and above, forwards and downwards. 

 That portion of the base of the hook which is not incumbent is 

 somewhat thickened, and the terminal part of the free point 

 is curved gently upwards. 



The upper of the band-like appendages, which are placed 

 transversely above the base of the hook, is the stronger and 

 broader of the two, and it has an irregularly undulating 

 border. The points of these appendages are bent down- 

 wards, rather over the hooks, and are truncated obliquely 

 from without to within. The surface is slightly plicated. 

 The upper and under borders of the appendages are also fre- 

 quently thickened, and the latter border is continuous with 

 a broad, apron-like fringe, which gradually becomes very 

 thin, and has a narrow edging, or hem, on both its lateral 

 margins, and corresponds in form to the space between the 

 hooks into which it is inserted on either side. As the hooks 

 are curved on their flat surfaces towards each other in the 



