12-: 



INVEETEBEATE ANIMALS. 



tary canal iii many Mammals, birds, and fishes. About one hundred 

 and twenty species of Echinorhynchus are known, but it is doubtful 

 if any mature example has been found in the human subject. 



Order II. Gordiacea. 



The Gordiacea, or " Hair-worms," are elongated, worm-like, or 

 hair-like Scolecids, in which the alimentary canal is without an 



anus, and the sexes are in separate 

 individuals. 



The best known representative of 

 this group is the common Hair- 

 worm {Gordius aquaticus, fig. 77) of 

 Europe and North America. This 

 curious worm is often found in 

 ponds and rivers, and may be two 

 or three feet long when mature. 

 Other examples are found coiled up 

 round the intestine of ground-beetles 

 or grasshoppers ; but it is not clear 

 whether these are in a natural posi- 

 tion, so to speak, or whether they 

 may not be wandered individuals. 

 In such cases, however, the worm 

 ultimately leaves its insect-host, and 

 betakes itself to water, where it lays 

 long chains of white eggs. The em- 

 bryo is at first free and locomotive, 

 with three pairs of cephalic hooks 

 (fig. 77, B), and a little protrusible 

 snout. It passes its next stage en- 

 cysted in the larvee of water-insects, 

 into which it has bored its way, 

 and then it gains access to the bodies 

 of fishes which eat these larvtv. Ultimately, however, it becomes 

 free, and makes its way to land. 



Fig. 77. — Gordiacea. A, A siDall in- 

 dividual of Gordius aquaticus, of 

 tlie natural size. B, Larva of Gor- 

 dius suhfurcaUii^, "with its piercing 

 proboscis and two rows of hooks, 

 enlarged. 



Order III. ]Nematoidea. 



The on\e.v Xenviioidea includes the typical " Round-worms '' and 

 " Thread-worms," some of which are parasitic, while others lead a 

 permanently free existence. The body is cylindrical, the integu- 

 ment being often annulated, but never ciliated. The mouth is 

 placed anteriorly (fig. 78), and leads through a gullet (g) to a strong 



