156 PARASITES OF THE DOMESTIC ANIMALS 



The classification adopted here places the smooth roundworms and 

 the annulated worms together in the phylum Ccelhelminthes, an ar- 

 rangement based upon the presence of a coelom or bod}^ cavity, which 

 is a structural feature clearly defuiing these worms from the Platyhel- 

 minthes and establishing a relationship between the smooth round and 

 annulated forms of primary importance. 



The Platyhelminthes includes worms which are flattened dorsoven- 

 trally, the two surfaces uniting in more or less sharp margins. There is 

 no body cavity, the various organs being embedded in a mass of con- 

 nective tissue and muscle fibers. The aHmentary tract is a simple or 

 bifurcated, sometimes branching, pouch having no anal opening (Fig. 85), 

 the inouth serving as both inlet and vent. In some parasitic 

 forms (t|ipeworms) alimentary organs are entirely wanting. 

 A true circulatory system is absent. There is a series of ex- 

 cretory tubes which ramify throughout the body, usually 

 opening to the outside near the posterior extremity. The 

 nervous system consists of ganglia located above the esoph- 

 agus, where this is present, and the lateral nerves which these 

 give off. Most all are hermaphroditic, the sexual organs 

 being distributed over a large portion of the body. 



As is true of the worms in general, free living forms are 

 found in fresh and salt water. They may often be revealed 

 clinging to the under side of rocks (planaria. Fig. 84) and 

 upon the moist soil, some of these specimens being nearly 

 transparent. The largest members of the division are the tapeworms. 

 ^^'hich may reach a length of thirty feet or more. 



The phylum contains two parasitic classes, as follows: 

 Class I. Trematoda. — The flukes. 

 Class II. Cestoda. — The tapeworms. 



Class I. Trematoda 



Platyhelminthes (p. 156). — All of the members of this group are 

 parasitic, living either as ecto- or entoparasites. The body is usually 

 leaf-like, often much like a pumpkin seed in form (Fig. 87), and is pro- 

 vided anteriorly with suckers by which attachment is made to the host. 

 In most of those entoparasitic (Distomese) two suckers are present, one 

 anterior and surrounding the mouth, and a second larger one just 

 posterior to the mouth on the mid-ventral line. In the ectoparasitic 

 species (Polystomese), which are usually parasitic upon the gills and 

 skin of aquatic animals, the suckers are more numerous. 



The alimentary tract leads by a short gullet to a bifurcation, forming 

 two elongated blind sacs which may or may not give rise to lateral 

 secondary pouches (Fig. 85). Eye spots occur in some of the ectopara- 

 sitic species and in the larvae of the entoparasitic. 



