NATURE AND ORGANIZATION OF PARASITES. 



analogous to stationary parasites. I only need to mention certain 

 caterpillars and other insect larvse, many of which lead a stationary 

 life like the intestinal worms, and, furthermore, resemble them in that, 

 in many cases (e.g., ichneumon -flies, &c.), they are occasionally or even 

 constantly parasitic. Besides these negative characters, stationary 

 parasites can in many cases be recognised by positive characters, such 

 as the possession of hooks and suckers, which serve to fix them on to 

 the body of their host. Structures of this kind are by no means con- 

 fined to stationary parasites, but are also commonly found in 

 temporary parasites, and occasionally even in free-living animals, 

 where, however, they are never so conspicuous or so constantly de- 

 veloped. The more the power of locomotion in a parasite diminishes, 

 the more difficult it is for it to migrate to another animal, and it must 

 therefore be provided with organs which will enable it to retain its 

 position under the most adverse circumstances. These organs of 

 attachment vary in character, in correspondence with the structure of 

 that part of the body of the host upon which the parasite dwells, 

 being generally stronger and larger in those forms which are parasitic 

 upon the outer skin than in those which live in the interior of the body 

 of their host. Among internal parasites, again, the organs of attach- 

 ment are generally more developed in those species which live in the 

 alimentary canal, since they have to withstand the pressure of its 

 contents. Many intestinal worms, however, 

 do not possess any hooks or other organs of 

 attachment ; but in these cases there is gene- 

 rally some compensation. Among the thread- 

 worms, for example, which we shall presently 

 consider, the form and length of the body 

 seem quite as fit to break the pressure of the 

 intestinal contents as to strengthen its hold 

 upon the intestinal wall. In Trickocephalus 

 (Fig. 3) the whiplash-like anterior part of the 

 body is actually imbedded in the mucous 

 membrane. 



In this case the form of the body in a 

 certain way makes up for the absence of 

 proper organs of attachment. When these 

 are present we find the greatest differences 

 in their structure and arrangement, which 

 correspond always to the needs and cir- 

 cumstances of the individual parasite. Some- 

 times, as in the flukes (Fig. 1), muscular suckers are present, which 

 work by atmospheric, or, more correctly speaking,by hydraulic pressure ; 



FIG. 1. Distomuni hi- 

 tcum (young), with suckers 

 and viscera (after de la 

 Valette). 



