vegetation at or near the water's edge. Thus larval 

 cysts can be ingested by water beetles, and perhaps 

 a falling water level exposes some of the vegetation, 

 making the cysts available to crickets and grasshop- 

 pers feeding near the water's edge. The genus Gor- 

 diiis is known from ponds and ditches all over the 

 world. Other genera are less cosmopolitan, but horse- 

 hair worms occur from the tropics to cold-temperate 

 regions, even above timber line on mountains. There 

 are about eighty species, only one of them marine. 



The Spiny-headed 



Worms 



( Phylum A canthocephala ) 



Only when they are tucked away in someone else's 

 intestine can these worms be looked on as animals 

 of unobtrusive habits, though it is true that few peo- 

 ple ever see any of the four hundred known species 

 or are even aware of their existence. The spiny head 

 referred to in both common and technical names is a 

 burrlike and retractile proboscis by which the worm 

 clings to the intestine of fresh-water, marine, or land 



vertebrates. Fishes and birds are favored hosts, but 

 many mammals receive their share of attention, and 

 occasionally also man. Like the tapeworms, which 

 they resemble in many respects, these spiny-headed 

 parasites have no mouth or digestive tract at any 

 time in life, risking all on finding hosts to support 

 them. They have few internal organs that are not di- 

 rected toward a prodigious production of offspring, 

 and the success of the species rests on enough of these 

 surviving all hazards and eventually making their 



way back to the vertebrate host to reproduce again. 



The adult lives a life of ease, absorbing food 

 through the body wall and resisting digestion by 

 means of the thin cuticle that covers the body. The 

 chief damage to the host is local injury at the point 

 where the proboscis is attached, but if the proboscis 

 perforates the wall, it may cause a fatal peritonitis. 

 In really heavy infestation the worms may interfere 

 with digestion and cause loss of appetite. 



The spiny proboscis, armed with rows of stout re- 

 curved hooks, can be turned inside out on retraction. 

 And the knoblike or slender forepart of the body, 

 made up of the proboscis and an unarmed neck at 

 the base of the proboscis, can be withdrawn into the 

 much larger trunk region. The trunk may be short 

 and plumpish or long and cylindrical, but only in 

 certain worms is it curved or coiled or beset with 

 spines. 



Most acanthocephalids are under 1 inch long, 

 some only a small fraction of an inch, but the com- 

 mon species that lives in pigs all over the world 

 reaches a length of more than 2 feet and looks as 

 formidable as its name, MacracaiUhorhynchiis hiriidi- 

 naceiis. This giant parasite has, in the past at least, 

 been reported in people of the Volga valley in south- 

 ern Russia. The knoblike proboscis is armed with 

 five or six rows of very stout thorns, and the long, 

 pinkish, wrinkled trunk tapers from front to rear. As 

 in nearly all spiny-headed worms, the male is much 

 smaller than the female. The eggs develop, within 

 the mother, into a young larval stage that is enclosed 

 in a hard spiny embryonic shell. Shed with the host's 

 feces, the shelled embryos can survive in soil for up 

 to three and a half years. When swallowed by grubs 

 of June beetles or similar insects, they develop within 

 the insect body. Pigs become infected when they eat 

 either grubs or beetles as they root about in pastures. 



The only other species that has been found at 

 times in man is Monilijormis diihiiis, a common par- 

 asite of house rats. In the United States and in South 

 America it spends its larval life in cockroaches, and 

 these infect rats that feed on them. In Europe a bee- 

 tle (Blaps) has been implicated as a larval host. The 

 adult worm may reach 1 foot in length and has a 

 really wicked-looking proboscis, cylindrical and cov- 

 ered with twelve to fifteen rows of thornlike hooks. 



People sometimes unwittingly eat cockroaches or 

 beetles, and there are other possibilities for getting 

 infected with these resourceful parasites, but fortu- 

 nately human infections are quite rare. The habits 

 of dogs provide more opportunities for such worms, 

 and dogs or coyotes in North America sometimes 

 harbor Oncicola canis and may display rabies-like 

 symptoms. In Texas, where most of the known cases 

 occur, the armadillo may act as a transport host be- 

 tween the dog and the arthropod that first harbors 

 the larva. 



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