CLASS VIII. PLATYELMIA: ORDER 3. CESTOIDEA. 



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ORDER 1. PLANARIDA. 



This order includes most of the free Platyehnia, 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 ratlier 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 Nemertidce, or Ribbon-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. TREMATOBA. 



These animals are all parasitic. One of the most noted species is the Fltjke, Distoma hepa- 

 j ticum, which infests the livers of sheep. Other species live in the intestines, the brain, and even 

 i the eyes of other animals. 



■ ORDER 3. CESTOIDEA. 



The general appearance of the animals belonging to this order is well shown in the 

 (r^\ ^-^ annexed fii^ure, which represents the 



i 



Common Tape-Worm, Ta;nia 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 which it an- 

 chors itself to the intestines of its vic- 

 tims. It has no mouth or digestive 

 oro-ans ; 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 Cmnurus ccrcbralis, 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-WORM. 



