Fig. 13. Free-swimming _Cercaria_, before the commencement of the formation of the cyst. Magnified 100 diams. ] [Illustration: Fig. 14. Diagram of the structure of a Turbellarian: _ng_, nerve- (cerebral) ganglia; _nb_, nerve-branches; _yg_, yolk-glands; _t_, testis; _o_, ova; _ov_, ovary; _c_, cirrus; _m_, mouth; _ph_, pharynx. Fig. 15. Diagram of a Nemertine: _b_, brain; _m_, mouth; _n_, renal organs; _id_, diverticula of intestine; _g_, gonads; _sn_, side nerve-trunk; _pr_, proboscis in its dorsal sheath. Fig. 16. Diagram of the structure of a Nematoid; _m_, mouth; _ph_, pharynx; _a_, anus; _o_, orifice of genital tube. ] NEMATODES (Thread-Worms or Round-Worms).—These are for the most part parasitic, and infest plants as well as animals; the common Round-Worms living parasitically in man (_Ascaris_, _Stronaylus_, _Trichocephalus_) belong to this Order. Sometimes they are parasitic in their early stages and later live a free life—such are _Gordius_ and _Mermis_. A specimen of a Mantid is exhibited from which half the body of the infesting _Gordius_ has already protruded (Fig. 17). One of the most remarkable _Gordii_ is the great elongated _G. fulgur_, or “Lightning Snake,” from Celebes. Another very large Nematode is the so-called Guinea-worm, or _Dracunculus medinensis_, which is found beneath the skin of the leg; it is very possible that this worm was the cause of the illness which afflicted the Israelites in their journey through the desert from Egypt to the Promised Land. [Illustration: Fig. 17. _Gordius_ escaping from a Mantid. ] [Illustration: Fig. 18. Figure of _Trichina spiralis_, showing the worms encysted in muscle. ] Of all Nematodes the most dangerous to man is the small worm which is known as _Trichina spiralis_ (Fig. 18); a series of models are shown which give a good idea of the structure of the female and the smaller male. The young make their way through the walls of the stomach of their host, and encyst themselves among its muscles: a piece of a sternothyroid muscle is shown, taken from a man in whose body it was calculated there were forty millions of encysted _Trichinæ_. Other Nematodes infesting man, such as _Filaria sanguinis hominis_, are too small for exhibition. Plants are not free from the attacks of Nematodes, and examples are shown, accompanied by an illustrating figure, of the Ear-cockle gall of wheat; this gall is due to the injuries inflicted by a minute Thread-worm—_Tylenchus tritici_. Wheat is, of course, by no means the only cultivated plant that is attacked by these minute worms; the history of most has, however, still to be made out. Holding a somewhat uncertain position in relation to the Round-worms are the parasitic _Acanthocephali_ (Thorn-headed Worms) and the free-swimming _Chætognatha_, or Bristle-jawed Worms; examples of both of these groups are shown, together with diagrams illustrative of their general structure.