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rounded particles. At this stage, also, incompletely developed 

 Cercarias may be seen in the perivisceral cavity. These Cer- 

 cariae are at first shapeless organisms, but after passing through 

 a series of gradations they ultimately assume a definite form, 

 which, in many cases, is sufficiently distinctive to enable us to 

 refer the Cercarise to particular species of Distoma. The older 

 writers regarded many of the cercarians as adult flukes. In 

 the early state these larvae are furnished with tails. They may 

 "be seen lodged within the cavity of the body of the sporocysts, 

 being twisted and folded in various attitudes. The Cercarise 

 not only exhibit a cephalic and ventral sucker, but also a dark 

 forked line representing the digestive system. At a still fur- 

 ther stage other structures come into view, until the perfect 

 Cercaria displays an oral sucker, a pharyngeal bulb, an 

 oesophagus, two alimentary caeca, a ventral sucker, a water- 

 vascular system consisting of two main excretory ducts, and a 

 contractile vesicle, by means of which the ducts communicate with 

 the external surface. The tail is conspicuous and furnished with 

 a fringe. The alimentary organs conform to the general trema- 

 tode type, but before passing into the sexually-mature condi- 

 tion other changes are undergone. The Cercariae part with 

 their tails, and subsequently they encyst themselves on or 

 within the surface of the body of some mollusk. Their pupa 

 condition is thus arrived at. The pupa itself differs from the 

 cercaria in presenting a double crown of hooks surrounding the 

 head, but the other organs correspond with those already 

 described. According to Yan Beneden the hooks make their 

 appearance immediately after encystation. In this condition it 

 is next transferred to the intestine of some higher animal, and 

 in this final situation it gradually acquires all those organs the 

 possession of which will entitle it to be called a sexually-mature 

 or adult distome. In the immature fluke we may now discern 

 the mouth, the buccal or cephalic sucker, the pharyngeal bulb, 

 the oesophagus, the digestive caeca, the coronal spines, the 

 contractile vesicle, the aquiferous system of vessels, the matrices 

 of the yelk-forming glands, and also a central mass of cellules, 

 from which all the other reproductive organs will in due time 

 be developed. In the adult Echinostoma militare the upper 

 third of the body is clothed with little spines. Taking this 

 example as illustrative of the ordinary mode of fluke develop- 

 ment we find that a change of hosts is necessary, and that in 

 the intermediate state they occupy the bodies of mollusks. 



