112 



INVERTEBRATE ANIMALS. 



of other animals. Thus the disease known as " hydatids " in 

 the human subject is caused by the presence in his tissues of 

 the cystic worms which are ultimately developed into the tape- 

 worm of the dog. 



ORDER II. TREMATODA (Gr. trema, a pore or sucker). 

 The " suctorial " worms, or " flukes," as the members of this 

 order are commonly called, are all internal parasites, inhabit- 

 ing various situations in different animals, but especially af- 

 fecting birds and fishes. They are all more or less flattened 

 and rounded in shape, and are furnished with one or more 

 suckers, by which they adhere. They are distinguished from 

 the Tceniada by always possessing an alimentary canal, which 

 is often much branched (Fig. 40, 1), is simply hollowed out 



FIG. 40. Trematoda. 1. Distoma hepaticum, the "liver-fluke," showing the branched 

 alimentary canal : 2. Anterior extremity of Distoma lanceolatum, enlarged ; a An- 

 terior sucker ; b Posterior sucker ; o Generative pore ; d Gullet ; e e Bifurcating aliment- 

 ary canal (after Owen). 



of the tissues of the body, and is never provided with a dis- 

 tinct anus. The best known of the Trematoda is the common 

 liver-fluke (Distoma hepaticum, Fig. 40, 1), which inhabits 

 the gall-bladder or ducts of the liver in sheep, and is the cause 

 of the disease known as the rot. In form it is ovate, flat- 

 tened on the two sides, and presenting two suckers, of which 

 the anterior is perforated by the aperture of the mouth. A 

 branched water-vascular system is present, and opens pos- 

 teriorly by a small aperture. The alimentary canal bifurcates 

 shortly behind the mouth, the two divisions thus produced 

 being much branched, and terminating posteriorly in blind 

 extremities. In Distoma lanceolatum (Fig. 40, 2) the intes- 

 tine is divided into two branches, but these are simple tubes, 

 and are not branched. 



