•280 ZOOLOGY sect. 



mature or strobila condition, but in that of the cysticercus. This 

 is Twnia cchinococcus, the presence of which produces what is termed 

 h} datid disease (p. 2S2). The adult Tccnia echinocomis is a very 

 small tape-worm with only three or four proglottides, occurring in 

 the intestine of the domestic Dog. The eggs passing out with a 

 liberated proglottis in the fagcos, may reach the alimentary canal of 

 Man uninjured in drinking-water, on the surface of salad vegetables, 

 and the like ; and, the egg-shells becoming dissolved, the containcil 

 hooked embrjos bore their way to the liver or the lungs or some 

 other organ. Arrived at its final destination, the embryo develops 

 into a cyst, which may become of enormous size. In the interior 

 of the 'irrinidry or mother-cyst are developed a number of sccondarj/ 

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

 externally, are formed very numerous scolices in the way already 

 described (p. 282). 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 — 

 c.(/., the Cysticercus celluloscc of Tcenicc solium. 



The most primitive of the Platyhelminthes are, without doubt, 

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

 must look for the nearest existing relatives to the Coelenterata. 

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

 and Ctenoplaiia (p. 22.5) 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, 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 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 Tui-bellaria 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 — 



