206 



MESOZOA THROUGH ENTOPROCTA: 



Figure 12.9 Life cycle of a free-living flatworm, Euplanaria (Dugesia): 

 A, mature adulf laying egg-filled cocoons in fresh-water habitat; B, 

 cocoons upon on object; C, immature flatworm emerging from a cocoon; 

 D, adult feeding; E, asexual cycle in which on adult constricts transversely 

 into two ports, each port regenerating into an adult; F, mating of two 

 hermaphroditic adults (internal cross-fertilization occurs). 



tion, it is essentially a process of healing rather than 

 of reproduction. 



CLASS TREMATODA (Flukes) 



Diagnosis: strictly parasitic; body undivided, 

 covered by a protective layer, a cuticle, one or more 

 suckers, but no epidermis or cilia; mouth typically 

 anterior; mostly hermaphroditic, with sexual re- 

 production; development direct (from egg to minute 

 adult form) in external parasites, normally indirect 

 in internal parasites and includes changes in hosts 

 (Figure 12.10). 



The flukes all live in or on other animals. A few 

 of them cause no apparent harm; all others are true 

 parasites. Associated with their mode of life are 

 various modifications of the basic structure and life 

 history. The mouth is anterior and leads into the 

 "throat" which gives rise to an intestine composed of 

 two arms and many side branches; the extent of the 

 digestive system greatly increases the intake of 

 nutrients. At least one attachment organ, a ventral 

 sucker is present. The external ciliated epidermis 

 characteristic of free-living forms is replaced by a 

 protective, noncellular cuticle. Especially striking 

 are the enlargement of sexual organs and the complex 

 life cycle. 



In the life cycle of Fasciola hepatica, the sheep liver 

 fluke, fertilized eggs leave the sheep in excrement. If 

 the eggs then reach water, they hatch into a ciliated 

 larva. The larva must enter a particular kind of snail 

 host to survive. It might be added here that almost 

 without exception snails are necessary intermediate 

 hosts for flukes. In boring into the snail's tissues, the 

 ciliated larva loses its ciliated epidermis and trans- 

 forms into a sac-like stage. Within the sac balls of 

 cells accumulate and differentiate into many in- 

 dividuals of the third immature stage. These third- 

 stage immatures are set free into the snail's tissues. 

 By the same processes and within each third stage, 

 fourth-stage immatures form. The fourth stages leave 

 the snail and encyst on grass. When the grass is 

 eaten, the cysts open in the intestine and young flukes 

 emerge. Boring out through the intestinal wall of the 

 susceptible animal, the flukes reach the body cavity 

 and migrate to the liver, where they bore into this 

 structure and take up their final position in the bile 

 ducts. 



From the above discussion we can see that para- 



Figure 12.10 Life cycle of a human liver fluke, Opisthorchis sinensis: 

 A, mature hermaphroditic adult in human liver bile ducts; B, egg case, 

 which contains a ciliated larva when it leaves the humon digestive tract; 

 C, ciliated lorvo, or miracidium, which enters an aquatic snail; D, sporo- 

 cyst, which develops from o miracidium (note thot sporocyst contains 

 many of the next stage); E, redia (like sporocysts contains the next stage); 



F, cercario, which leaves the snail and swims to ond enters a fish's tissues; 



G, metacercaria, the encysted stoge within the fish. When mon (espe- 

 cially in Asio) eats raw or poorly cooked fish, the adult emerges and 

 takes up its habitat in the human bile duct. 



