104 G. WITENBERG 



abomasum, where they perform the last mouh, but only a part 

 of them emerge immediately afterwards into the intestinal tract. 

 The majority remains in the mucosa for as long as eight weeks, 

 and some of them eventually die in this site without emerging 

 from the nodules which have formed around them. 



The physiological significance of this phenomenon is not 

 quite clear (there are nematode larvae which develop perfectly 

 without it). Among several explanations proposed, it has been 

 stressed-^ that this 'temporary burrowing' may have an evo- 

 lutionary significance, suggesting a step in the direction of a 

 more compUcated migration via the circulary system as is 

 characteristic of the genera Ascaris and AncyJostoma; alter- 

 natively, it may be regarded as a relict of such a migration. 



The larvae of the horse nematode, Strongylus vulgaris are 

 another example of retarded histotropic phase. During their 

 development they accumulate in the root of the anterior mesen- 

 teric artery where they may stay for a long period and undergo 

 only slight changes. The route by which the larvae reach this site 

 is a matter of controversy. Their presence inside the artery is 

 connected with the formation of aneurism and thrombosis which 

 may cause grave consequences to the host. 



HYPOBIOSIS OF WORMS DUE TO HYPOBIOSIS OF THE HOST 



As a rule, adult parasitic worms live in uniform and constant 

 conditions inside or upon their hosts and their life goes on 

 uninterruptedly. In some instances, however, these conditions 

 change and the parasites mostly perish. In rare cases, the parasitic 

 worms are able to adapt themselves to new conditions. The most 

 striking instances of such change occur during hibernation or 

 aestivation of some vertebrates. Only a few relevant observations 

 have been recorded. Van Beneden-^ and Markova-- observed 

 changing relationships in bats, Dubinina in frogs and land 

 tortoises23. 24 j^ appears that the reaction of the parasites to the 

 changing physiology of the host is diff'erent in different species. 

 The trematodes become letharsic until the hosts have returned 



