PHYLUM PLATYHELMINTHES 1 57 



Order 3. Polycladida (Fig. 104). Marine Turbellaria with 

 a central digestive chamber which gives off many lateral branches 

 (D). Examples: Stylochus and Leplo plana. 



3. Class II. Trematoda 



a. The Liver-fluke — Fasciola hepatica 



The liver-fluke is a flatworm which lives as an adult in the 

 bile ducts of the liver of sheep, cows, pigs, etc., and is occasionally 

 found in man. Figure 105 shows the shape and most of the ana- 

 tomical features of a mature worm. The mouth (O) is situated 

 at the anterior end and lies in the middle of a muscular disc, 

 the anterior sucker. A short distance back of the mouth is the 

 ventral sucker (S) ; it serves as an organ of attachment. Between 

 the mouth and the ventral sucker is the genital opening through 

 which the eggs pass to the exterior. The excretory pore lies at 

 the extreme posterior end of the body, and another pore, the 

 opening of Laurer's canal, is situated in the mid-dorsal line 

 about one third the length of the body from the anterior end. 



The digestive system is simple. The mouth (Fig. 105, O) opens 

 into a short globular pharynx which leads into another short 

 tube, the oesophagus. The intestine consists of two branches, 

 one extending from near the anterior to the posterior end on 

 each side of the body. Many small branches (Fig. 105, D) are 

 given off from the intestine as in Planaria (Fig. 99, i), and no 

 circulatory system is therefore necessary for the transportation 

 cf food material. 



The excretory system is similar to that of Planaria (p. 153), but 

 only one main tube and one exterior opening are present. The 

 nervous system also resembles that of Planaria (Fig. 99, en, In). 



The suckers are provided with special sets of muscles enabling 

 them to fasten the animal to its host. Three layers of muscles 

 lie just beneath the ectoderm: (1) an outer circular layer, (2) a 

 middle longitudinal layer, and (3) an inner diagonal layer. 



The body of the liver-fluke is triploblastic. The ectoderm is a 



