90 THE TREMATODA 



Dactylocotyle and other ectoparasitic forms would soon be carried 

 away by the current of water passing over the gills of the fish ; 

 and Amphistomum, or other internal parasites, would be driven down 

 the alimentary tract of its host, by the passage of food, unless 

 they were able to adhere in some way to the host ; the modifica- 

 tion of the musculature of the body wall is the simplest method 

 of adhesion. 



But suckers occur, though of a very simple kind, in many 

 Poly clad and several Triclad Turbellaria, and large ones in Temno- 

 cephala, so that even these characteristic organs are not in reality 

 novelties or peculiar to the class. It does not appear possible to 

 regard the suckers of Trematodes and Turbellaria as truly homo- 

 logous ; they are rather homoplastic, for they vary in position and 

 relation in both groups. Indeed, but for the absence of cilia, there 

 is no essential difference between a Trematode and a Turbellarian, 

 and there is little difficulty in deriving the former from some 

 Rhabdocoelous form of the Turbellaria, which had taken to the 

 habit of temporarily associating itself with a definite animal, as 

 the Triclad Bdelluridae do at the present day. As this habit 

 became fixed, the means of attachment became improved, resulting 

 in a single posteriorly placed sucker. The animal was thus able 

 to live permanently on its host, and having a ready supply of food 

 at hand, in the host's blood and mucus, took to sucking it, for 

 which purpose a second sucker in the neighbourhood of the mouth 

 would aid the parasite during the use of the " pharynx bulbosus," 

 which acts as an aspirator. The two characteristic suckers having 

 been developed, each became modified in different directions. The 

 anterior sucker became double in the ectoparasitic forms ; the pos- 

 terior sucker became more elaborate, or booklets were developed 

 to aid in adhesion. 



It is customary to look on the Malacocotylea (Digenea) as more 

 highly developed than the Heterocotylea, and as probably derived 

 from them. This view, no doubt, depends on the endoparasitic 

 habit of the former order, and on the fact that the host is a 

 vertebrate, and that the life-history is a complicated one. But 

 Looss is of opinion that, on the contrary, the Heterocotylea are 

 the higher order. But if the anatomy of each order and of 

 Rhabdocoelida be compared, organ for organ, with one another, 

 we shall have to take a middle position and look upon the two 

 orders as diverging at a very early stage in phylogeny. 



Very possibly some TemnocephcUa-likQ form was the ancestor 

 of the Trematoda a form, as said above, with a posterior sucker, 

 but without the anterior ones ; the intestine was sac-like, and no 

 doubt the genital organs posterior to it. 



The assumption of ectoparasitic and endoparasitic habits, with 

 the anterior suckers or sucker differently arranged, led to a diver- 



