TKEMATODA 



35 



CLASS II. TREMATODA 



The class TrSmato'da is comprised of worms either internally 

 or externally parasitic. The body is usually thicker than that 



of the turbellarians. The 

 form is usually leaf-like, 

 though it is sometimes elon- 

 gated. The anterior end is 

 distinguished by the arrange- 

 ment of suckers, and, in some 

 of the external parasites, by 

 eyes. 



Fig. 23. The common liver-fluke 

 (Fasci'ola hepat'ica) enlarged to show 

 the anatomic characters : ac, Acetab- 

 ulum; c, p., cirrus pouch; i, intestinal 

 ceca; m, mouth with oral sucker; ov, 

 ovary; p. b., pharyngeal bulb; s. g., 

 shell gland; t, profusely branched 

 testicles; ut, uterus; va, vagina; 

 v. g., profusely branched vitellogene 

 gland. (After Stiles, 1894, p. 300.) 



Fig. 24. Embryo of the com- 

 mon liver-fluke (Fasciola hepatica) 

 boring into a snail x 370. (After 

 Thomas, 1883, p. 285.) 



The suckers are organs of adhesion and are sometimes armed 

 with bristles or hooks. They are also used in locomotion, which 

 is a sort of looping, like that of the leech. Except in two cases 

 the vibratile cilia are not found on the surface. 



