The _Ascaris acus_ of the pike lives at first in a common white 241 fish, the _Leuciscus alburnus_, and passes with this fish, which serves it as a vehicle, into its final host. Another common nematode, the _Oxyurus vermicularis_ (Fig. 69), a parasite of man, is a small worm of the size of a fine pin, which often multiplies in the rectum of children, causing intolerable itching. It is by means of their microscopic eggs that they penetrate into the system; these are hatched in the stomach, and are completely developed at the end of eight or ten days. They pass from the anus in great numbers. [Illustration: Fig. 68.--Trichocephalus of man.--1, female, _a_, cephalic extremity, _b_, caudal extremity and anus, _c_, _d_, digestive tube and ovary, _e_, orifice of sexual apparatus. 2, isolated egg. 3, male, _a_, cephalic extremity, _b_, anus, _c_, digestive tube, _d_, spicula or penis, _e_, sheath into which it is withdrawn.] [Illustration: Fig. 69.--Oxyurus vermicularis.--1, male of natural size, 2, female, id., 3, cephalic extremity, magnified.] The brood of worms from the eggs of the _Ascaris megalocephala_ of the horse live in freedom, and go through all their phases until their sexual development separately; there are males and females. The generation which descends from these is distinguished by being of a 242 much smaller size. The name of _Trichocephalus_ has been given to nematodes which have the cephalic extremity very thin, and ending in such a fine point that it is difficult to discover the mouth. The Trichocephalus of man (Fig. 68) is a curious nematode, which was discovered by a student at Göttingen, in 1761. It is usually found in the cæcum, in which more than a thousand have been met with together. The female is from 40 to 50 millimètres long, the male about 37 millimètres. A female _Trichocephalus affinis_ having laid her eggs in an aquarium, the whole of the contents were introduced into the stomach of a lamb, seven months afterwards, and the walls of its intestines became infested with trichocephali. No animal at any time has attracted so much attention as that little worm which lives in flesh, rolled up; it is about the size of a millet seed, and was found by chance in the dissecting-room of a London hospital, some forty years ago. The plague and the cholera did not inspire so great fear, and this fright had almost passed from Germany throughout the rest of Europe. We were not among those who wished to take measures at all hazards against the invasion of this worm, since nothing induced us to believe that more trichinæ existed then in Belgium than in ordinary times. These measures would have produced no other effect than uselessly to disturb the minds of the public.