STRUCTURE OF PARASITES 



201 



Structure of Parasites. — It is very certain that all 

 existing parasites have descended from free-living, non- 

 parasitic ancestors. Where the parasitic habit has been 

 of comparatively short duration, or where the activities 



Fig. 50. — Adult tape-worm, Muliiceps multiceps, from the intes- 

 tine of the dog. (Natural size; after Hall.) 



of the parasite are confined to the skin of the host, the 

 effects on the parasite of such a mode of life are not es- 

 pecially marked. The mosquito, for example, flies from 

 host to host, and possesses organs of locomotion and sensa- 

 tion of the usual insect type. On the other hand there 

 are numerous types that are wholly dependent upon 

 one host, and these are often so 

 highly modified that their rela- 

 tionships are difficult to deter- 

 mine. In the Sacculina, an 

 animal related to the barnacles 

 and parasitic on a crab, the body 

 is scarcely more than a bag filled 

 with eggs and attached to a root- 

 like absorbing system pene- 

 trating the tissues of the host. 

 In the tapeworms, that cling to 

 the wall of the digestive tract and absorb digested 

 materials through their body wall, the organs of loco- 

 motion and digestion have entirely disappeared. In the 



Fig. 51. — Encapsuled 

 trichinae in trunk muscle of 

 pig. (Greatly magnified; 

 after Braun.) From Kellogg 

 and Doane, Econ. Zool. 



