AFFINITY OF TREMATODES TO PLANABIANS. 109 



would be difficult to distinguish such forms from ectoparasitic Trema- 

 todes. But this ciliated coating is lost as soon as the parasitism 

 becomes stationary or permanent, and the change of the host takes 

 place only during the larval period. 



After these observations, the relationship of the Trematodes to the 

 free-living Planarians may be taken as established, so that I may omit a 

 comparison of the young forms of these two groups. I will only men- 

 tion that the above described (p. 30) peculiar developmental relations 

 of the embryos of Monostomum mutabile occur also in certain worms 1 

 closely related to the Planarians, perhaps even in the Planarians 

 themselves. Likewise the fact that the embryos of the entozootic 

 Trematodes often leave the egg without a differentiated intestine, and 

 sometimes (namely, when they develop into the so-called " sporocysts," 

 Fig. 49, p. 71) never possess such an organ, will hardly seem peculiar 

 in creatures resembling the Planarians. It has been proved that there 

 are forms among the free-living Planarians which are devoid of a proper 

 intestine (Acoela, Oulian.), its place being occupied by a readily move- 

 able mass of protoplasm, which absorbs the nutriment that passes in 

 through the mouth, as is well known to be the case in the Infusoria. 



The absence of an intestine in the internal parasites is thus not in 

 all cases the result of a retrograde development, but, under certain 

 circumstances, also the sign of an imperfect differentiation ; and this 

 is the case not only in the embryos of the above-mentioned Distomidse, 

 but also in those of the tape-worms, in which it is impossible to find 

 even the rudiment of an intestine. 2 This is a further proof that these 

 latter animals are far more completely adapted to a parasitic life than 

 the other related parasites. This is much more strikingly shown in 

 the Tseniadse, however, than the Bothriocephalidae, by the fact that the 

 former do not even possess the embryonic ciliated coating which is 

 seen in the young forms of the latter (Fig. 70), as in the Trematodes, 3 

 and which, as in these, subserves the function of free locomotion. The 



(as had been supposed to be the case by me in 1848) to the Nemertines, a group closely 

 related to the Planaridse. 



1 In this connection, see the observations concerning the so-called "Desor's Larva," 

 Max Schultze, Zeitschr. f. wiss. ZooL, Bd. iv. } p. 179, 1853; and Krohn, Mutter's Archiv 

 f. Anat. u, Pliysiol., p. 293, 1858, and especially Barrois, " Mdm. sur 1'embryologie des 

 Nemertes,".47m. Sci. nat., se>. 6, t. vi., p. 1, 1877. 



2 Huxley considers this circumstance so important, that it causes him to doubt the 

 origin of the Helminths without intestine from animals with intestine ; and he throws out 

 the suggestion that they may be independent of free forms, and be directly and continu- 

 ously developed forms, that were from the commencement parasites without intestine. 

 See " Anatomy of Invertebrated Animals," pp. 213, 652, 675 : London, 1877. 



3 In many cases also among the Trematodes, and even Distomidae, the embryos are 

 without a ciliated coat. Von Willemoes Suhm classes among the 28 known embryos of 

 Trematodes 10 non-ciliated forms (Zeitschr. f. wlss. ZooL, Bd. xxiii., p. 339, 1873). 



