"220 ENTO-PARASITES AND HYGIENE. 



the same measure as they lead a more stationary life 

 (Pilaria, Gordius, Mermis, etc.). Still more rudimentary 

 are the muscles in the Trematods and Cestods, which in 

 many cases can execute only very slow contractions of 

 the body. However, even here many differences occur, 

 according to circumstances. 



This degeneration of the locomotive and sensitive ap- 

 paratus does not remain without influence upon the ner- 

 vous system. In many cases we can observe how the re- 

 duction of one system is strictly parallel to that of the 

 other. Only a short time ago a nervous apparatus 

 seemed to be missing in many worms; now we know 

 that it must be present, though its actual existence has 

 not yet been proved. 



Those parasites living in the interior of their hosts get 

 ahead of their exterior colleagues on account of their 

 floating in a nutritious liquid, so that they need not have 

 any special organs for taking up food and digesting it, as 

 the ecto-parasites must have. . The latter must necessarily 

 be provided with borers, stings, saws and other instru- 

 ments to tap the host in order to get near its blood or 

 lymph. The ento-parasites live in a place where they 

 always have their food ready for use without any special 

 endeavor on their part. And yet there are a few of them 

 which possess some armament around their mouths, es- 

 pecially when they do not live chiefly on the liquids in 

 which they float, but on blood, which they must cause to 

 flow. There is especially one family of Nematods, the 

 Strongylids, which live in that way; they possess an ex- 

 tremely firm, horny mouth, on the rim of which may be 

 seen a number of chitin hooks, that are thrust into the 

 mucous membrane of the alimentar}'^ canal. Through 

 powerful sucking these parasites are able to cause 

 a good deal of blood to flow, and examination reveals 

 the fact that their intestines are always tightly filled 

 Avith blood. If such parasites exist in great numbers in 



15S 



