114 INVERTEBRATE ANIMALS. 



worm-like in shape, by the fact that the alimentary canal is 

 furnished with a distinct anus, and by the absence of an ex- 

 ternal opening to the water- vascular system of the adult, in 

 some cases at any rate. Their development sometimes shows 

 phenomena very similar to what occurs in the Echinodermata, 

 the larva (Fig. 41, 4) being a free-swimming, ciliated organ- 

 ism, of which only a portion is employed in producing the 

 adult animal, the remainder being cast off as useless. 



OEDER IV. ACANTHOCEPHALA (Gr. aJcantha, thorn ; Tee- 

 phale, head). The "thorn-headed worms" included in this 

 order are all internal parasites. They are worm-like in shape, 

 marked with transverse wrinkles, and destitute of any mouth 

 or alimentary canal. The anterior extremity of the body forms 

 a kind of proboscis or snout, which is armed with recurved 

 hooks, and has placed at its base a single nervous ganglion. 

 Beneath the skin is a net-work of canals, containing a clear 

 fluid, and believed to represent the water-vascular system. 

 The thorn-headed worms include some of the most formidable 

 parasites with which we are as yet acquainted, the best known 

 being the various forms of JSchinorhynchus, which are found 

 inhabiting the alimentary canal in many mammals, birds, and 

 fishes, but not as yet in man. 



ORDER V. GORDIACEA. The Gordiacea, or "hair-worms," 

 are thread-like parasites which in the earlier stages of their 

 existence inhabit the bodies of various insects, chiefly beetles 

 and grasshoppers. They possess a mouth and alimentary 

 canal. The sexes are distinct, and they leave the bodies of 

 the insects which they infest to breed, subsequently deposit- 

 ing their eggs in long chains either in water or in some moist 

 situation. In form the Gordiacea are singularly like hairs, and 

 they often attain a length very many times greater than that 

 of the insect in which they live. 



ORDER VI. NEMATODA (Gr. nema, a thread). In this 

 order are the " round-worms " and " thread-worms," both of 

 which are parasitic, together with a number of worms which 

 lead a permanently free existence. All the Nematoda (Fig. 

 42) are elongated and cylindrical or thread-like in shape. 

 They possess a distinct mouth, and an alimentary canal which 

 is freely suspended in an abdominal cavity, and which termi- 

 nates in a distinct anus. They possess a system of canals 

 which are believed to represent the water-vascular system ; 



