CLASS YIII. PLATYELMIA: ORDER 3. CESTOIDEA. 
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ORDER 1. PLANARIDA. 
This order includes most of the free Platyelmia, which are of an oval or elliptical form, and 
are very commonly furnished with an extensible proboscis. They are of a gelatinous consistency, 
and can reduce their whole substance to the form of a lump of jelly, in which condition they 
occasionally force themselves rather disagreeably upon the notice of some incautious water-cress 
eaters. They inhabit both salt and fresh water, where they swim about rapidly by an undulating 
movement of the body, in the manner of the leech, and creep with great ease upon stones and 
aquatic plants. They are generally of small size, but exceedingly voracious. 
In South America, Dr. Darwin observed some terrestrial animals which approached the Plan- 
arida very closely in their characters ; they lived among rotten wood, upon which they appeared 
to feed, and were marked on the back with stripes of bright colors. 
The family of Nemertidm, or Bihhon- Worms, is composed of animals with elongated, ribbon- 
like bodies, possessing a protrusible proboscis, and sometimes attaining a length of fifteen feet. One 
species, the Borlasia, is found on the coasts of England and France. This formidable worm lies 
coiled up beneath stones during the day, but goes about at night in search of its prey. 
ORDER 2. TREMATODA. 
These animals are all parasitic. One of the most noted species is the Fluke, Distoma hepa- 
ticum, which infests the livers of sheep. Other species live in the intestines, the brain, and even 
the eyes of other animals. 
' ORDER 3. CESTOIDEA. 
The general appearance of the animals belonging to this order is well shown in the 
annexed figure, which represents the 
Common Tape-Worm, Tcenia solium, 
of the human intestines. The body is 
composed of numerous joints or seg- 
ments, each one resembling the others ; 
these are often several hundreds in 
number, and the animal sometimes at- 
tains a length of upwards of ten feet. 
It is furnished with hooks and suckers 
at the head, by means of wdiich it an- 
chors itself to the intestines of its vic- 
tims. It has no mouth or digestive 
organs ; so that it obtains its nourish- 
ment by absorption through the skin. 
One of the most curious facts in regard 
to this species is, that in different ani- 
mals it assumes entirely different forms. 
In one state it is called Coenurus cerehralis, and is found in the brains of sheep, and often 
inflicts very great injury. 
The reproduction of this species is equally remarkable. Each segment of the body is provided 
with male and female sexual organs ; when these have reached maturity, the segment is cast off 
to seek a new place in which its ova may be developed. To aid in this process the joints, when 
cast off, are endowed with a considerable power of motion, and will live for several days when 
placed in favorable circumstances. Thus it appears that nature has manifested the same care 
and displayed the same ingenuity in providing for the wants and in maintaining and multiply- 
ing the species of these hideous and destructive worms, as in the highest forms of animal life. 
THE COMMON TAPE-WOEM. 
