t infests the small intestines, and seldom migrates into
the stomach or large bowel. Instances are recorded, however, in which it
has crept upward in the esophagus, larynx, nostrils, and eustachian
tube; but their presence in these parts is of comparatively rare
occurrence, and is generally caused by some local irritation which
compels their migration. The fact that they have been found in the
peritoneal sac, gave rise to the opinion that they perforate the
intestine; but careful observations have proved that they can only
escape through openings made by ulcers.

This species has been found in adults, but is more common in children
from three to twelve years of age. The number of this species existing
in a human body is variable. Sometimes only two or three are found. At
other times a hundred, and even twice that number, are voided in a few
days.

(2.) The _ascaris vermicularis_, thread, pin, or seat-worm, is round,
very slender, and about half an inch in length. The habitation of this
species is the rectum, and they are often found matted together in the
excrement. They are very active, even after ejection, and have been
known to cause great local irritation by entering the vagina and
urethra. Their presence is an occasional cause of masturbation. It is
impossible to estimate the number of these parasites that may exist in
the human rectum. Great numbers, sometimes, are voided at a single
evacuation.

(3.) The _tricocephalus dispar_ is a third variety of the round worm,
and is said to infest the bodies of almost every species of mammalia. As
its name indicates, the upper portion of its body is slender, hair-like,
and terminates at the lower extremity in a thick, spiral portion. It is
from one to two inches in length, and is found attached by its head to
the mucous membrane of the caecum, and, in rare instances, in the colon
and small intestine. They are rarely numerous.

_Taeceniae_ or _tape-worms,_ are hermaphrodites, of a flat, ribbon-like
form, and are composed of numerous segments, each of which is provided
with a complete set of generative organs, and contains ova for the
production of thousands of individuals. Some authors have supposed that
each segment, or joint, is a distinct individual, but the existence of
one head for the whole precludes this theory. There are two species of
_taeniae_ developed in the human intestine; the _taenia solium_ and the
_taenia lata_.

(4.) The _taenia solium_ is the species