80 THE TREMATODA 



degrees, so that ultimately the original form of the body is lost 

 (Fig. XIIL). 



The posterior sucker or the cotylophore in the Heterocotylea 

 is frequently armed with booklets or spines, aiding the worm in 

 fixation to its host; whereas booklets are never present in this 

 organ in the Malacocotylea, or in Aspidocotylea, a fact which 

 struck both Burmeister and Monticelli. Moreover, in the Poly- 

 stomidae the suckers are strengthened by a special development 

 of the cuticle to form a " chitinoid " skeleton (the substance is 

 soluble in 35 per cent KHO, according to Cerfontaine), (Fig. XIX. 

 5). In some forms the sucker, when in use, retains its cup shape, 

 when it may be termed " acetabulate," or it becomes folded 

 across its middle, like the two valves of a lamellibranch shell, 

 holding on to the host like a pair of forceps, when it may be 

 termed " valvate " (Fig. XIX. 4). 



The muscles which constitute the sucker present some variety 

 in their arrangement (Goto). For instance, in Calicotyle y three 

 sets of muscles are distinguishable : (a) radial along the ridges, 

 derived from the longitudinal muscles of the body ; (b) circular 

 muscles round the margin ; (c) dorso-ventral fibres traversing the 

 substance of the sucker from the dorsal body wall (Fig. XIX. 3). 



On the other hand, the wall of the suckers in Polystomidae 

 consists of muscular fibres, arranged at right angles to the surface, 

 in between the chitinoid skeleton, and limited, bodywards, by a 

 distinct membrane; these may be considered" intrinsic"; while 

 muscle fibres from the general somatic musculature are attached 

 to the skeleton, and on their contraction the floor of the apparatus 

 is raised, and the sucking action produced (Fig. XIX. 6). In 

 valvate suckers other muscle fibres are attached to the various 

 pieces of the skeleton, serving as occlusor, divaricator, arid con- 

 strictor muscles. 



It is an interesting fact that in Monocotyle (Goto) and in Diclido- 

 phora (Cerfontaine) the intrinsic muscles are transversely striated. 



Apart from the existence of an elaborate adhesive apparatus, 

 which is foreshadowed by the sucker of some Polyclads and 

 Triclads, the Trematodes differ from the Turbellaria in only one 

 essential particular, and that is in the nature of the outer cover- 

 ing of the body ; for with the parasitic habit the cilia of the 

 ancestral Platyhelminth have gone, and the whole body is covered 

 by a thick, firm "cuticle," or "investing membrane" (Wright). 

 There can be little doubt but that this cuticle which occurs 

 also in Cestoidea and Nematoidea has been developed in rela- 

 + 'on to the parasitic life, and serves as an efficient protection 

 against the action of digestive or other secretions of the host. In 

 the Heterocotylea and Aspidocotylea this cuticle is comparatively 

 simple ; in the Malacocotylea it is frequently armed with minute 



