266 ZOOLOGY 



SECT. 



or daughter-cysts, and from the walls of these, both internally and 

 externally, are formed very numerous scolices in the way that has 

 already been described (p. 262). Hydatid cysts are very common in 

 some domestic animals Oxen, Sheep as well as in Man. Various 

 other Cestodes occur in the bladder-worm stage occasionally in 

 Man e.g. the Cysticercus celluloses of Tcenia solium. 



The most primitive of the Platyhelminthes are, without doubt, 

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

 are to look for the nearest existing relatives to the Coelenterata. 

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

 and Ctenoplana (p. 212) 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 Coelenterata is 

 shown, pefhaps, 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 Planarians, 

 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 tin.- 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 polyphyktic group 

 i.e., 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, others, again, nearer the Triclads, in the majority of 

 their characters. 



The remarkable life-history of the Digenetic Trematodes is, as 

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

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

 genctic and parthenogenetic one (heterogeny). The sporocyst and 

 redia are to be looked upon as intercalated stages as cercarise 

 which exhibit paedogenesis. The cercaria is the characteristic 

 larval stage of the Trematodes, and corresponds to the cysticercus 



