PERMANENT PARASITISM 3 



]. G. H. Zeder, J. G. Bremser, K. A. Rudolph! 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 (Plaiyhelminthes, 

 Xemat helminthes). 



PERMANENT PARASITISM in the course of time has caused 

 animals 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 the 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, like many decidedly temporary parasites, has lost 

 its wings a characteristic of insects in consequence of parasitism, 

 yet nobody would deny its insect nature ; such also occurs in other 

 temporary parasites (Cimex, Pulex). On the other hand, the changes 

 in a number of permanent ectoparasites (such as parasitic Crustacea) 

 are far more considerable, and correspond with those that have 

 occurred 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 Crustacea. 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. To all appearances, this character is not secondary 

 and recently acquired, but represents a primary character inherited 

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

 descendants, because of its use during the larval stage (e.g., the larvae 

 of a great many Trematodes, the oncospheres of some Cestodes). 

 Amongst the retrogressions, the loss of the organs of sense may 

 be mentioned, particularly the eyes, which are still present, not only 

 in the nearest free-living forms but also in the free-living larv?e of 

 true parasites. It is only quite exceptionally that the eyes are 

 subsequently retained, as a rule they are lost. Lastly, in a great 

 many cases the digestive system also disappears, as in parasitic 

 Crustacea, in a few nematodes and trematodes, in all cestodes and 

 Acanthocephala. There remain at most the rudiments of the muscles 

 of the fore-gut, but these are adapted to entirely different uses. 



