THE TREMATODA 89 



gested that the canals absorbed water through the skin and passed it out 

 through the pore ; or (6) as lymphatic, or vascular, in part at least ; even 

 Villot, 1882, takes the view that it serves for excretion, absorption, respira- 

 tion, and circulation. The main course of the canals was known to Laurer 

 and to Mehlis (1831). The fact that the bladder and canals form part of 

 one system was first pointed out definitely by P. J. van Beneden (1852). 

 The movement of the contained fluid was caused, according to Ehrenberg 

 (1835), by valve-like folds, endowed with the power of oscillation ; but in 

 the next year v. Siebold rightly showed that it is due to the action of cilia. 



The finer structure was investigated by Fraipont (16), who described 

 the " flame cells," but believed them to be in communication, not only 

 with the tubules, but also with the spaces in the parenchyma ; this has, 

 however, been shown to be erroneous. 



The brain and the main nerves were first accurately localised by 

 Eamdolir ; but Otto, 1816, contended that the vitelline ducts, longitudinal 

 and transverse, were the true nervous system, and described ganglia at 

 their junction. Our knowledge of this system is due primarily to Lang 

 (24) and Gaffron (17). 



The hermaphrodite nature of the Trematodes appears to have been 

 recognised by Miiller ; but for a long time the ventral sucker was inter- 

 preted as the "birth pore," till Nitzsch (1819) discovered that the sucker 

 is imperforate. The penis was known to Eudolphi, and the common 

 genital pore was held to belong only to the male ducts. 



V. Siebold was the discoverer of the fact that the egg-producing organ 

 is distinct from the yolk-forming gland. 



Relations of the Group Parasitism. It is almost an axiom that 

 parasitism leads to degeneration of the parasite, and this usually 

 in an extreme degree ; but in the Trematodes this degeneration 

 is scarcely recognisable ; for beyond the absence of the ancestral 

 locomotor organs, viz. the cilia of the outer surface, it is scarcely 

 possible to point to any sign of degeneration common to the group. 

 Degeneration of the sense organs is another characteristic of para- 

 sites, and although eyes are frequently present in the Heterocotylea, 

 they are less elaborate than in the majority of Turbellaria ; and in 

 Malacocotylea they are absent, except in the free-living miracidium 

 and cercaria. The nervous system shows no essential difference 

 from that of Turbellaria ; though in the Malacocotylea the peri- 

 pheral stems and commissures are more definitely arranged. 



The generative organs, too, agree with those of many Rhab- 

 docoele Turbellarians, and the alimentary canal is well developed in 

 all members of the group. 



The cuticle, as has been suggested, has been developed in 

 relation to the parasitic habit, and Temnocephala forms an interesting 

 link between the Turbellaria and Trematoda in this respect. 



The suckers so frequently associated with parasitism, and so 

 eminently characteristic of the group, seem to be the cause rather 

 than the result of parasitism, for such relatively small animals as 



