lflll l Watson: The Genus Gyrocotyle. 389 



are seen to be composed of a finely felted mass of fibres, circular 

 in general direction, embedded in a homogeneous interstitial sub- 

 stance which is acted on by the macerating agent (which also 

 removes the cuticula from the body) (pi. 37, fig. 33). Each spine 

 is set in a pit lined witb a very thin cuticula of felted fibres I par. 

 filt, pi. 37, fig. 34) which passes indistinguishibly into the fibres 

 of the surrounding parenchyma. The spine is surrounded by 

 muscle-bundles, which serve to protrude and retract the spine. 

 The retractors are musclediundles from the outer transverse 

 layer of the body musculature; they insert at the base of the 

 spine (pi. 37, fig. 30). The protractors are bundles of fibres 

 inserted at the base and for some distance on the sides of the 

 spine ; they originate or are attached to the cuticular musculature, 

 principally to its transverse layer. 



The direction of the spines is shown in the figures (.pi. 34. 

 figs. 10-13). In general they may be said to be directed anter- 

 iorly and laterally. This is the most advantageous arrangement 

 possible for maintaining the attachment of the parasite to its 

 host; the position of the greatest number of spines on the ventral 

 surface and around the rosette also further this end. For loco- 

 motion, the spines seem to be arranged to hinder rather than 

 help; hut they are probably so completely retracted that their 

 rounded ends offer no special resistance to the forward move- 

 ments of the worm. At all events, however theoretically difficult 

 their arrangement and direction render locomotion, the worm 

 does move, and with considerable rapidity, in spite of them. 

 Naturally, ease of locomotion is a secondary and firmness of 

 attachment a primary consideration in the economy of this 

 creature. 



III. Musculature. 



The musculature of Gyrocotyle is exceedingly well developed, 

 and composed of powerful, neatly balanced sets of muscles. It 

 constitutes the great bulk of the body. This is not surprising, 

 when the extreme contractility and general activity of the worm 

 is considered. 



In general, muscles are grouped in pairs running at right 

 angles to each other. There is throughout the body, but espe- 

 cially prominent in the region between the acetabulum and the 



