INTESTINAL WORMS. 177 



the exception of those of touch about the mouth in certain thread 

 and suctorial worms, such as threads, papillae, 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 1 . 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 Echi- 

 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 Tcenia, 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 

 EcMnorhynchus 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 Ccenurus. Of 

 the genus Ascaris all the classes of Vertebrates have many species; 

 of Strongylus especially the mammals. The four genera, Distoma, 

 Tcenia, 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 gigantcnm. 

 VOL. T. 12 



