266 VERMES. 



anterior so-called cephalic ganglion or brain, from which the nerves 

 radiate anteriorly and posteriorly. In the segmented forms the 

 branches running back are united into a single ventral cord, which 

 forms a special nerve ganglion in each segment. The sexual organs 

 exhibit, in the flat-worms at least, a very considerable development. 

 They consist of male and female parts, which are sometimes in 

 separate individuals, but are also often, and perhaps more frequently, 

 united in the same body. 



The development is as a rule associated with a metamorphosis. 

 In many groups there is an alternation of generations. Thus we dis- 

 tinguish the so-called " nurses," which arise directly from the eggs, 

 and the "sexual animals," which originate partly by budding, and 

 partly from egg-like cells in the interior of the " nurses." 



Of all the divisions of the animal kingdom, that formed by the 

 worms is by far the most important for our present purpose, for to it 

 belong the great majority of parasites, and these the most dangerous. 



In man the parasitic worms inhabit exclusively the internal 

 organs, where they are soaked by the juices of their host. Only on 

 aquatic animals do wo find them as external parasites. Tlieir ecto- 

 parasitism on land animals is rendered impossible by the necessary 

 conditions of respiration (see p. 13). 



The parasitic worms infesting man and the higher animals are 

 therefore literally intestinal worms (Entozoa). They have a true 

 right to this name, which is often used in a wider sense to denote all 

 parasitic worms, including even ectoparasites. 



At one time these worms were regarded as forming a single united 

 group, not only with a biological, but also v/ith a systematic signifi- 

 cance ; in other words, they were erected into a special class of 

 Entozoa or Helminths. So is it according to Guvier, for example, and 

 also according to many more modern zoologists — e.g., Owen, v. Siebold, 

 van der Hoeven, &c. Cuvier even had the idea of separating this 

 class of Entozoa from the worms proper {i.e., from the ringed worms 

 (Annelida), which he had united with the likewise ringed Arthropoda 

 into a single group, Annulata), and thought of uniting them with 

 the Eadiata. 



Now, however, zoologists are of a different opinion on this point. 

 Not only do they insist on the union of the Helminths with the 

 higher worms, but deny the propriety of a special class of intestinal 

 worms in any natural system of the animal kingdom. 



Even the adherents of the older opinion must at any rate confess 



that the aforesaid class " includes most diverse animals " — so diverse, 



indeed, that no common character in their organization is to be found. 



In reality the limits of the class are simplv those of a common mode 

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