PERMANENT PARASITISM 3 



cussed in the same sense as one speaks of land and water animals, 

 i.e., without conveying the idea of a classification in such a grouping. 

 It is true that formerly this was universally done, but very soon the 

 error of such a classification was recognised : still, until the middle of 

 last century, the helminthes were regarded as a systematic group, 

 although C. E. v. Baer (1827) and F. S. Leuckart (1827) strenuously 

 opposed this view. Under the active leadership of J. A. E. Goeze, 

 J. G. H. Zeder, J. G. Bremser, K. A. Rudolfi and F. Dujardin, the 

 knowledge of the helminthes (helminthology) developed into a special 

 study, but unfortunately it lost all connection with zoology. It 

 required the intervention of Carl Vogt to disestablish the helminthes 

 as one class of animals, by uniting the various groups with those of 

 the free-living animals most closely related to them (Platy helminthes, 

 Nemathelminthes). 



PERMANENT PARASITISM has, in the course of time, caused 

 creatures adopting this mode of life to undergo considerable, some- 

 times even striking, bodily changes, permanent ectoparasites having 

 as yet undergone least alteration. The latter sometimes bear so 

 unmistakably the likeness to that group to which they belong that 

 even a superficial knowledge of their structure and appearance 

 often suffices for the recognition of their systematic position. For 

 instance, though the louse has, like many decidedly temporary 

 parasites, lost a characteristic of insects its wings in consequence 

 of parasitism, yet nobody would deny its insect nature (Cimex, 

 Pulex). On the other hand, however, the changes in a number of 

 permanent ectoparasites (such as parasitic Crustacea) are far more 

 considerable, and correspond with the changes that have taken 

 place in permanent endoparasites. 



These alterations depend, partly on retrogression and .partly on 

 the acquisition of new peculiarities ; in the former case the change 

 consists in the loss of those organs which have become useless in a 

 permanent parasitic condition of existence, such as wings in the 

 louse, and the articulated extremities seen in the larval stage of 

 parasitic crustaceans. The loss of these organs goes hand in hand 

 with the cohesion of segments of the body that were originally 

 separate, and alterations in the muscular and nervous systems. In 

 the same manner another means of locomotion is lost the ciliated 

 coat which is possessed by many permanent parasites during their 

 larval period, and which is, to all appearances, not secondary and 

 recently acquired, but represents a primary character inherited 

 from free-living progenitors, and still transmitted to the altered 

 descendants, because of use during the larval stage (the 



