70 FLATWORMS, ROUNDWORMS, AND ROTIFERS 



of the host. The number of larvae that may be encysted, 

 at one time, in the muscles of a human being is enormous. 

 It has been estimated that, by eating an ounce of "trichi- 

 nosed" pork not well cooked, eighty thousand worms might 

 be set free in the intestines. 



Prevention. — Thoroughly cook every bit of pork eaten. 

 If practicable, feed only vegetable products. If swill 

 containing scraps of pork is fed to hogs, always cook it 

 thoroughly before feeding. 



Rotifers, or Wheel Animalcules 



The third branch which we discuss in this chapter is 

 represented by the miscroscopic animals known as roti- 

 fers, or wheel animalcules. These interesting animals are 

 abundant in fresh-water ponds, pools, puddles, and streams, 

 and a few occur in the sea. They are almost sure to be 

 met in aquaria. The majority are free-swimming, but a 

 few are fixed in the adult state. Although these animals 

 are microscopic they have distinct digestive and nervous 

 systems and a complex body structure. 



Figure 38 shows the form and appearance of a rotifer 

 common in fresh water ponds. The body has two regions 

 — a broad portion, the trunk, which forms the greater 

 part of the body, and a rather long, slender portion, 

 the tail. The trunk is inclosed by a glassy, trans- 

 parent cuticle. At the anterior end of the body is an 

 organ known as the trochal disk. The edge of the disk is 

 fringed with ciha, which by their pecuhar motion give the 

 appearance of a wheel rotating. This disk, with the ciha 

 and apparent rotary motion, is a distinguishing feature of 

 these animals and gives them their name, Rotifera. The 

 anterior portion of the body may be withdrawn into the 



