282 ZOOLOGY SECT. 



some of the simplest Turbellaria, and it is among these that we 

 must look for the nearest existing relatives to the Ccelenterata. 

 In none, however, is the relationship very close. Cceloplana 

 and Ctenoplana (p. 221) are probably rather to be looked upon 

 as Ctenophores specially modified in accordance with a creeping 

 mode of progression than as intermediate forms between Cteno- 

 phores and Turbellaria. The relationship with the Ccelenterata is 

 shown, perhaps, most strikingly when we take into account the 

 development of the Turbellaria, in the earlier stages of which there 

 is to be recognised a marked tendency towards a radial symmetry. 

 In their development the Turbellaria, that is to say the Polyclads, 

 show some special points of resemblance to the Ctenophora ; the 

 ectoderm cells are formed and spread over the rest in a similar 

 way, and the bands of cilia have a disposition and mode of move- 

 ment that strongly bring to mind the ciliary swimming plates of 

 the Ctenophora. But though there is much to be said in favour 

 of the view that the Turbellaria and the Ctenophora were derived 

 from a common, not very distant, stock, the latter are too 

 specially modified to be looked upon as the direct ancestors of 

 the former. 



The connection between the Turbellaria and the Monogenetic 

 Trematodes is very close so much so that it is difficult to give 

 any characters of universal occurrence distinguishing all the 

 members of the two classes. The Trematodes are, in fact, to be 

 looked upon as Turbellaria some of whose external characteristics 

 and, in the case of the Digenetica, whose life-history have been 

 specially modified in accordance with a parasitic mode of life. It 

 is not unlikely that the Trematodes may be a polyphyletic group- 

 ie., that different families may have become developed from 

 different families of Turbellaria altogether independently, some 

 of them appearing to be nearer the Rhabdocceles, others nearer 

 the Polyclads, and others, again, nearer the Triclads, in the majority 

 of their characters. 



The remarkable life-history of the Digenetic Trematodes may, 

 as already pointed out, be looked upon as a special form of alter- 

 nation of generations the alternation of a sexual with a pcedo- 

 genetic and parthenogenetic generation (heterogeny). The sporocyst 

 and redia are to be regarded as intercalated stages as cercarise 

 which exhibit paedogenesis. The cercaria is the characteristic 

 larval stage of the Trematodes, and corresponds to the cysticercus 

 or cysticercoid of the Cestode. The most important difference 

 between these is in the presence in the former of an enteric cavity, 

 and its absence in the latter. There seems to be something to be 

 said in favour of the view that the enteric cavity of the cercaria is 

 represented by the frontal sucker of some scolices, and by the 

 rostellum of the majority. 



Between the adult Cestodes and the Trematodes an intimate 



