214 ^^^^ ANIMAL KINGDOM 



Adhesive 

 orOan. 



Mouth 



Midvcntral 

 SucKer 



Posterior 

 SucKe-r 



Ante-rior 

 SucKer 



A. Mono^&nea. B. Digc-nea. 



Figure 11.11. The two major groups of flukes, class Trematoda. 



Cuticle 



Circular muscle 

 Longitudinal mascle 

 Diagonal inuscle 



Mesenchyme 

 ce-Us 



Mesenchyme 



Figure 11.12. Part of the body wall of a trematode. Note that an epidermis is 

 missing, and that the covering cuticle lies directly on tissue of mesodermal origin. 



involves modification of the suckers and replacement of the epidermis 

 by a hard cuticle, apparently secreted by the underlying mesodermal 

 tissue. They feed on the slime, on epithelial cells, and on blood extrud- 

 ing from wounds they make in the skin of the host. 



The Digenea include a number of medically important parasites, 

 such as the liver flukes, lung flukes and blood flukes. In some regions 

 of the world, especially in Asia and the Southwest Pacific, whole pop- 

 ulations of people are kept in constant poor health by a single species 

 of digenetic fluke. These are endoparasitic worms, living inside the 

 body of the host. They have an anterior sucker surrounding the mouth 

 (Fig. 11.11) and a large midventral sucker. Adults usually mate, but 

 if one individual is alone in its host it can undergo self-fertilization by 

 autocopulation. In one group, the blood flukes, the sexes are separate. 

 These live in the circulatory system in pairs, the more slender but 

 longer female nestled in a ventral groove of the male. As in the Mono- 

 genea, digenetic eggs are laid one to a capsule, but the capsules are 

 often retained in the parent until they are ready to hatch. 



The life cycles of this group are quite complex. The egg hatches 

 into a ciliated larva, the miracidium (Fig. 11.13), which invades the first 

 host, usually a snail. The miracidium has a well developed brain and a 



