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684 close scrutiny, be made out. In the first place, it can be seen that the entire animal is enclosed in an exceedingly delicate, limp, structureless sack, in which it moves backwards and forwards (Fig. 97, a). This sack, or "sheath," as it is generally called, although closely applied to the body, is considerably longer than the worm it encloses; so that that part of the sack



which for the time being is not occupied is collapsed and trails after the head, or tail, or both, as the case may be. It can be seen also that, about the posterior part of the middle third of the parasite, there is what appears to be an irregular aggregation of granular material which, by suitable staining, can be shown to be a viscus of some sort (Fig. 97, b). This organ