INTESTINAL WORMS. 1G 
the exception of those of touch about the mouth in certain thread 
and suctorial worms, such as threads, papille, or tentacles, which 
can be extended by eversion. 
A special muscular system is usually present, but the muscular 
fibres are not always united into bundles to form distinct muscles. 
In the suctorial worms the muscular fibres in their entire course are 
completely united and interwoven with the rest of the mass of the 
body!. In the tape-worms fibres are visible beneath the skin 
running longitudinally. In the thread and thornheaded-worms two 
layers of muscular fibres may be distinguished beneath the skin, 
the one longitudinal, the other transverse. The proboscis of Kchi- 
norhynchus has special muscles for its inversion and production. 
The Trematodes make use of their suckers, especially the pos- 
terior, for fixing themselves. Other intestinal worms have spines 
or hooks for that purpose, which are movable, and often possess 
considerable hardness. It is probable that these hooks cause by 
their irritation an increased afflux of fluids, whereby the nutrition 
of the worm within the animal that harbours it is facilitated. 
Entozoa are inhabitants of very different classes of animals; the 
most numerous are the species which occur in birds and fishes. 
Amongst the tape-worms especially are genera, which are found 
exclusively or principally in fishes; nearly all the species of the 
genus Bothriocephalus occur in fishes; of Tenia, on the other 
hand, more than half the numerous species occur in birds, very 
many in mammals, in fishes very few. Of the genera Distoma and 
Echinorhynchus so rich in species, by far the most of these are found 
in fishes and birds. The genus Cysticercus appears to occur almost 
exclusively in mammals, as also Echinococcus and Cenurus. Of 
the genus Ascaris all the classes of Vertebrates have many species; 
of Strongylus especially the mammals. The four genera, Déstoma, 
Tenia, Ascaris and Echinorhynchus, contain the greatest number of 
the known species of intestinal worms, especially the three first. 
Entozoa live in all parts of the body, but mostly on mucous mem- 
branes, in the intestinal canal and the lungs. Even in the heart 
and the blood-vessels some species occur. 
1 See DIESING, op. cit. Tab. xxi. figs. 4—8 in Amphistoma giganteum. 
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