14 



OCCURRENCE OF PARASITES. 



the -latter, moreover, live in comparatively similar circumstances, 

 while the conditions of entoparasitism are most varied. 



It must not be forgotten, however, that there are a few entoparasi- 

 tic Arthropoda belonging to the Insecta and Arachnida. The most strik- 

 ing example is furnished by the Pentastomida (Fig. 5), which, during 

 the early stages of their existence, inhabit the internal organs of both 

 terrestrial and aquatic animals, and on this account were included by 

 the earlier helminthologists among the Helminths proper. A closer in- 

 vestigation has shown that this classification, though hardly to be 

 wondered at, is erroneous ; the Pentastomida are undoubtedly to be re- 

 ferred to the Arachnida, but they differ from them by the entire absence 

 of lungs, and in this respect approach the intestinal worms. Many 

 mites also (Fig. 6) possess no respiratory organs, and agree with the Pen- 

 tastomida in breathing by means of the skin ; this is facilitated by their 

 small size, which implies a relatively large surface, and by the fact that 

 they are usually to be found in damp situations, sometimes imbedded 

 in the epidermis (Sarcoptes}, almost entoparasitic, sometimes upon the 

 hairy portions of the skin (Dermatodectes, &c.). But these instances 

 must not be considered as proving that all entoparasitic Arachnida and 



FIG. 5. Pentastomum denticulatum from 

 the liver of man. 



FIG. 6. Sarcoptes srablti. 



Tnsecta differ from their immediate allies by the absence of respiratory 

 organs. On the contrary, the majority possess the normal tube-like 



