THREAD-WORMS. 125 
being smaller and shorter than the females. With the ex- 
ception of the skin and lemnisci, all the parts of the adult 
worm, the nervous and reproductive systems as well as the 
beak, originate in the primitive 
rudimentary digestive cavity, 
appearing as rounded masses of 
cells of like size, but differing in 
structure histologically. With 
the growth of the beak begins 
the development of the repro- 
ductive apparatus, and the hooks 
are simply modified cells, with 
the outer surface chitinized. 
Order 2. Nematodes.—The first 
suborder of this group, compos- 
ing the true round worms, is re- 
presented by Ascaris, Oxyuris, 
Trichina, etc. The human 
round worm, Ascaris lumbri- 
coides Linn. (Fig. 83), is re- 
markable for its large size, and 
may be recognized by its milk- 
white color, as well as by the 
three papille around the mouth. 
It inhabits the intestines, some- 
times the stomach and cesopha- 
gus, and has been known to per- 
forate the walls of the intestine. 
The species of Ascaris are very 
numerous, infesting mammals, 
and especially fish, often occur- 
ring encysted in the flesh of the 
cod and other edible salt and — Fig. 83.—Ascaristumbricoides. « 
fresh water fish, but are as a jared’ at hae neice 
rule harmless. Ascaris mystax WE nHeaby teetolie, uke’ male, 
lives in the intestines of the pai a fee of eet 
cat. 
The common pin-worm lives in the rectum of children. 
It is the Oxyuris vermicularis Linn. (Figs. 84, 85). The 
