TREMATODA 



215 



developed. The function of these muscles is evident from their 

 position ; the meridional fibres flatten the suctorial disc and diminish 

 the depth of its cavity, so that the internal surface may adhere to 

 the object to be held ; if the equatorial fibres now contract, the 

 sucker rises by elongating longitudinally, and its inner surface is 

 drawn in by the contraction of the radial muscles. Thus the 

 sucking disc becomes adherent. Usually also there is a sphincter 



O.s 



G.p. 



Ex. p. 



FIG. 121. Harmostomum leptostomum, Olss., an immature 

 specimen from Helix hortensis. Nervous system, according to 

 Bettendorf. A.s., ventral sucker; C.g., cerebral ganglion; 

 Ex. p., excretory pore ; G.p., genital pore; O.s., oral sucker; 

 M.d., dorsal medullary nerve ; M.I., lateral medullary nerve ; 

 N.ph., pharyngeal nerve ; Af.v., ventral medullary nerve. 

 Magnified. 



at the border of the suckers, which plays its part during the act of 

 adhesion by constricting in a circular manner that part of the mucous 

 membrane to which it is attached. The loosening of the fixed sucker 

 is effected by relaxation chiefly of the radial fibres, by the contraction 

 of the meridional fibres and certain bundles of muscles situated at 

 the base and at the periphery of the suckers. The connective and 

 elastic tissues between the muscles of the suckers probably also take 

 part in the process. 



