HIE SCIENCE OF LIFE. 



while the Tape-worm of the dog develops from cysts 

 found in the hare, and that of the cat from cysts in the 

 mouse ; most cases requiring two animals as hosts for 

 perfecting the growth of the worm. (Fig. 1 18.) 



Order 2. Trematoda ; the Flukes. (Gr., trema, a hole.) 

 These are flat or roundish parasitic worms. The intes- 

 tine is branched, and, as in Coelenterata, there is but a 

 single opening, which serves for both mouth and anus. 

 There are suckers at the anterior end of the disk. They 



are met with sometimes in 

 the liver of the sheep. 

 Order 3. Turbellaria. 

 These are non- parasitic, 

 and may be found on the 

 sea-shore, under stones, or 

 in fresh-water pools, or on 

 moist ground. They are 

 small, ciliated, and flat 

 worms, which glide with a 

 slug-like motion over wet 

 surfaces, or swim by the 

 vibrations of their cilia. 

 In the small flat Plana- 

 rians the digestive cavity 

 is greatly branched. (Fig 

 1 19.) In others it is a sim- 



II9 .-Structure of Polycelis levigatus, 

 (Planarian worm.) 



ple pouch, with no 



^ rifice ' In the 

 forms It is elongated. Some 



of the largest (the Nemerteans] are like long ribbons; 

 sometimes, as in Borlasia, being twelve feet long. 



