56 



LECTURE V. 



Dist. hepaticurn' 

 nat. size. 



1 am compelled to limit ray illustrations of the anatomy of this 

 order almost to the two species which infest the human subject ; 

 these are the Distoma hepaticurn {Jig. 27.) and the Distoma lanceo- 

 latum (Jig. 28.). Both are peculiar to the biliary 

 ducts and gall bladder, but may pass thence into the 

 intestine. Both, likewise, are more commonly found 

 in the ordinary domestic animals, as the sheep and ox? 

 than in the human subject. 



A full-grown Distoma hepaticurn is of a flattened, 

 ovate, or oblong-ovate form, broader and rounded 

 anteriorly, attenuated posteriorly ; from ten to sixteen 

 lines in length, from four to seven in width % the broad 

 end sends forward a sort of conical neck or head, 

 convex above, flat below ; one of the suctorial acetabula {a) is at the 

 extreme apex of this process^ a little turned downwards ; the other (6) 

 is at the under part of the body, at the base of the neck : the first sucker 

 is perforated by the mouth ; the posterior and larger one is imperforate, 

 and serves merely as an organ of adhesion. You will observe, also, a 

 small depression {d) between the characteristic suckers, in which the 

 genital pores are placed : not unfrequently the curved or spiral penis 

 may be observed projecting from the anterior of these orifices. 



The body is of a whitish yellow colour, variegated near the margins 

 by the yellow ova, and on the dorsal aspect by the brown colour 

 of the double ramified alimentary tube. The integument is soft : 

 traces of muscular fibres can hardly be discerned, except around the 

 larger subventral sucker. 



Dr. Mehlis, who has given the best anatomical account of the 

 human Trematoda^ describes and figures the nervous system of the 

 Distoma hepaticurn as a delicate oesophageal filamentary ring, with a 

 slight ganglionic enlargement on each side, from which minute fibres 

 pass into the suctorial sphincter ; and two large filaments pass back- 

 wards, one on each side, as far as the ventral sucker. 



I have tested this description by a dissection of the largest known 

 species of Distoma, the Dist. clavatum, whose anatomy I have de- 

 scribed in the Zoological Transactions. You may distinctly perceive 

 in this dissection the oesophageal nervous circle, the small cephalic 

 filaments, and the two widely separated nervous chords of the trunk. 

 In this specimen also, you will see the integument raised as a distinct 

 membrane from the outer transverse muscular fibres, and a portion 

 of these is reflected from the inner longitudinal stratum. Feeble 

 analogues of these parts of the muscular system are doubtless pre- 

 sent in the smaller Distomata of the human subject. 



The sole aperture of the alimentary system is that of the anterior 



