1911] Watson: The Genus Gyrocotyle. 391 



lie the inner transverse muscles, a very heavy sheet of fibres 

 running at right angles to the longitudinal axis, around the body. 

 These fibres also are continued for some distance into the region 

 of the lateral folds, but are considerably reduced. These muscle 

 layers constitute the peripheral musculature and arF found 

 throughout the whole body, though less and less strongly devel- 

 oped the further laterad they pass. 



Within the inner transverse muscles lie the vitellaria, the 

 large excretory canals, the central nervous system, the glands 

 of the reproductive system, and the strongest muscles of the 

 body, the inner longitudinal. In the region of the body anterior 

 to the testes and posterior to the ovaries, these muscle-bundles 

 occupy the whole of the space inside the vitellaria. In the inter- 

 mediate region, especially in the region of the uterus, the layer is 

 considerably reduced, and pushed to one side. It is quite thick 

 ventrally, but there are very few fibres dorsal to the uterus. 

 These longitudinal fibres insert on the acetabulum, and around 

 the neck of the rosette and on the walls of the canal. It is by 

 the contraction of these powerful muscles that the worm moves 

 about, thrusts forth and around and withdraws the proboscis- 

 like acetabulum, and firmly attaches the posterior rosette. 

 Furthermore, as Lonnberg pointed out, this muscle layer being 

 absent in the lateral frills, and the outer longitudinal fibres being 

 very weakly developed there, on contraction of the longitudinal 

 muscles the lateral folds are increased in amplitude. Lonnberg 

 regards the frills as due wholly to contraction of the body mus- 

 culature ; the writer, in agreement with Wagener, regards them as 

 independent structures, existing in all states of expansion of the 

 animal. 



The musculature of the acetabular region (pi. 40, fig. 43) 

 shows plainly the origin of the acetabulum as an invagination of 

 the anterior extremity of the body. There is a doubling of the 

 layers, giving twelve instead of six. By inverting the anterior 

 extremity we get a mass of predominantly longitudinal muscles, 

 corresponding to the inner longitudinal body-layer, on the out- 

 side of the acetabulum. These fibres are meridional with refer- 

 ence to the acetabulum as a whole and constitute more than half 

 its bulk. Inserting along the surface of this mass are the inner 



