THE TREMATODA 79 



I 



The mouth was probably at first employed as a sucker for 

 adhesion to the host during the action of the pharynx, as it now is 

 in Monocotyle ; the next stage is exhibited by OnchocotyL, in which 

 special muscle fibres are developed around the mouth, so as to form 

 an indistinct "oral sucker," which has become much further 

 differentiated in the Malacocotylea. From this condition the 

 arrangement more usually met with in the Heterocotylea may 

 be derived where a pair of suckers are developed, one on each 

 side of the mouth, and communicating with the buccal cavity , 

 these are known as " buccal suckers," and are met with in the 

 majority of the Polystomidae. By the removal of these from the 

 mouth, they lose the connection with the buccal cavity, and a pair 

 of independent " lateral suckers " are formed, as in Tristomum. 



The posterior adhesive apparatus presents considerable variety. 

 No doubt the single sucker at the hinder end of the body represents 

 the primitive arrangement ; this sucker, which is always " simple," 

 and never armed in the Malacocotylea, is usually "multiloculate" in 

 the Aspidocotylea and Heterocotylea, owing to the special develop- 

 ment of muscular ridges, giving rise in the former order to trans- 

 verse and longitudinal, and in the latter to radial ridges, starting 

 from a circular ridge surrounding the centre of the sucker. 



It seems not improbable that the six or eight suckers of the 

 Polystomidae, arranged upon a caudal disc or " cotylophore," have 

 been derived phylogenetically by a further development of this 

 arrangement of muscle groups, till the loculi became entirely 

 independent. Finally, in the Microcotylidae, the presumed sub- 

 division of the sucker has gone very much further, resulting in a 

 considerable number of small suckerlets arranged on a membranous 

 cotylophore at the posterior lateral margins of the body ; this 

 apparatus must have been derived from the single sucker by the 

 cotylophore extending along each side, instead of remaining 

 terminal. 



In the Malacocotylea the primitively posterior sucker has 

 moved forwards in the Distomidae, so as to lie quite far forwards 

 in the ventral surface. But accessory adhesive organs are developed 

 to a greater degree, and in more varied form, in the Malacocotylea 

 than in the Heterocotylea. The papillae covering the ventral 

 surface of Homalogaster (Fig. X.) and Gastrodiscus are provided with 

 retractile tips, and aid in fixation ; they appear, indeed, to be 

 replacing functionally the posterior sucker, which is small in the 

 former, and quite minute in the latter genus. In the Monostomidae 

 this posterior sucker has disappeared, and fixation is effected 

 partly by the oral sucker, but chiefly by the retractile warts along 

 the dorsal or ventral surface of the body (Fig. X.). An accessory 

 organ of quite another type is developed in the Holostomidae, the 

 sides of the fore body being folded over ventrally in various 



