3420 THE WORM-LIKE ANIMALS 



everywhere in fresh water adhering to weeds, the edges of the wheel disc are pro- 

 duced into distinct bristle-bearing lobes; but in the allied Melicerta there is no such 

 production of the disc. 



The last order, Scirtopoda, comprises only the two genera Pedalion and Hexar- 

 thra, each of which is represented by a single species. The two resemble each other 

 and differ from all other rotifers in possessing three pairs of limbs, ending in a fan- 

 shaped tuft of setae. The body is conical with a broad square-cut head, furnished 

 with two wreaths of cilia and a pair of conspicuous eyes. In Hexarthra, which 

 bears a strong superficial resemblance to the Nauplius larva of some crustaceans, 

 the three pairs of limbs spring from the ventral surface, the first pair being con- 

 siderably the largest and the third the shortest; but in Pedalion they are arranged 

 round the body in pairs, one limb projecting from the middle of the back, another 

 from the ventral middle line and two from each side, the ventral limb being the 

 largest of the six. By means of these appendages the creatures are able to project 

 themselves through the water in a series of jerks. Pedalion has been discovered in 

 various parts of England, and Hexarthra in brackish water in Egypt. The male 

 of Pedalion is a veritable dwarf as compared with the female, the body and limbs 

 being greatly reduced in size, and the latter merely represented by three stumps, 

 each of which terminates in a pair of long bristles. 



THE THREADWORMS OR ROUNDWORMS Class Nematohelminthes 



These worms are characterized by having a thread-like body, covered with 

 tough, elastic integument, but usually showing no distinct traces of being divided 

 into segments like those of leeches and earthworms, and possessing no trace of 

 limbs. The sexes are generally distinct. The group is divided into the three orders 

 Acanthocephali, Nematoidea, and Chsetognatha. 



SPINY-HEADED THREADWORMS Order ACANTHOCEPHALI 



In this order is contained the single genus Echinorhynchus, which is appropri- 

 ately named for animals possessing a protrusible proboscis, armed with several rows 

 of backwardly-directed spines. The chief characteristic in which this order differs 

 from the next is the lack of any special alimentary canal and digestive apparatus. 

 In the adult stage the species of Echinorhynchus are found in the intestines of verte- 

 brates; the large species figured on p. 3421 (E.gigas} infesting the pig. But in order 

 to reach its final residence in this host, it has to spend its early days in the grub or 

 maggot of the cockchafer and allied beetles. These latter are rooted up, and de- 

 voured by pigs, which thus unconsciously swallow the worm. Similarly, E. proteus 

 of various fish lives in an immature state in the intestine of the water shrimp, which 

 swallowed it while still in the egg; and E. moniliferus, which occurs adult in such 

 rodents as hamsters and voles, lives during the larval state in beetles. Another 

 species, E. polymorphus, has to be transplanted from the body of the water shrimp 

 into that of a duck to reach maturity. 



