198 WAGENER^ ON GYRODACTYLUS ELEGANS. 



is on the ventral aspect. The middle portion of the animal 

 is usually enlarged, the enlarged portion corresponding to 

 the situation of the uterus. Should this organ contain no 

 ovum, nor any embryo, the vesicular elevation is produced by 

 a clear fluid with which the uterine cavity is distended. 



The cephalic lobes are separated from the root from which 

 they spring by a shallow groove on the ventral aspect, which 

 leads to and ends in the mouth. Frequently, also, a groove 

 exists on the ventral surface, close above that border of the 

 caudal disc which is unfurnished with hooks. The borders 

 are continuous with each other, in the middle line of the 

 body. 



The external integument presents no definite structure. 

 Occasionally, in certain states of contraction of the animal, 

 very delicate transverse lines, formed of extremely minute 

 points, might be seen crossing the surface at regular in- 

 tervals, especially in the caudal portion. Around the mouth, 

 when the eight pharyngeal papilla are protruded, fine longi- 

 tudinal folds are often visible on the ventral aspect of the 

 cephalic extremity. Below the mouth these folds become 

 more and more distant from each other, and gradually dis- 

 appear. 



Muscular system. — Three parallel longitudinal lines may be 

 seen at the border of the body of a Gyrodactylus. The external 

 and middle of these lines belong to the integument. The space 

 between the middle and inner lines is rather the narrower, 

 but equally transparent with the other. The innermost 

 line represents the outer border of the visceral mass, in 

 which delicate longitudinal lines may often be remarked, 

 which appear to radiate into the caudal disc ; and may, 

 probably, be regarded as representing muscular fibres. A 

 radial striation is very obvious in the caudal disc itself. 



Similar fine lines may be seen entering and terminating 

 in the lateral process or appendage of each hook. In the 

 same way the transparent substance inclosing the central 

 pair of hooks of the caudal disc exhibits striae running 

 parallel with its borders, and which are also, probably, to be 

 referred to contractile elements. 



The opening for the penis-like organ afterwards to be 

 described, Uke the sexual aperture of Octobothrium lanceo- 

 latum, is surrounded with minute booklets, from the base of 

 each of which two striae proceed, which are, probably, 

 muscular fibres. 



A very remarkable phenomenon may be seen in Gyrodac- 

 tylus, shortly after the birth of an embryo. Folds and club- 

 shaped villi arise over the whole surface of the animal, into 



