440 



ESSENTIALS OP ZOOLOGY 



sites, there are also a few endoparasitic species. For example, the 

 horse bot, Gastrophilus, which is the larva of a fly, is parasitic in the 

 stomach of horses ; long wormlike arachnids known as Linguatulids 

 or tongue worms are found in the intestines of some reptiles and 

 mammals ; and Sacculina, a crustacean, parasitic on crabs and lobsters, 

 sends rootlike outgrowths all through the body of its host, although 

 the saelike bodj^ remains on the outside. 



Some Representative Parasites 



Protozoa. — The very small amoeba-like protozoans of the genus 

 Endamoeha are examples of parasites only slightly modified for 

 parasitic life. There are two distinct stages in the life cycle, the 





... V- - 



•■•SlSB '••■.•. .••'•••■.'>l!T"Sl -•••'••'.•. V'..'- - 



■---nb.c. 



■ V-;.r 



cVtr.W- 



"^^Mi. 



vi 



^^■■■■ 



■"-n. 



Fig. 185. — Amoeba histolytica, one of the important protozoan parasites. It is 

 the causal agent of amoebic dysentery. A, Stained vegetative amoeba ; B, cyst with 

 four nuclei ; n, nucleus, showing peripheral chromatin granules and central karyo- 

 some ; r.b.c, ingested red blood corpuscles ; chr.b., chromatoid body. (Reprinted by 

 permission from Introduction to Human Parasitology by Chandler, John Wiley 

 and Sons, Inc., after Dobell. ) 



active form being much like a small amoeba except that the pseudo- 

 podia are shorter and move more slowly; these active forms iinaJly 

 round up and become surrounded by a semirigid, resistant cyst wall. 

 In this encysted condition the Endamoeba is passed from the host 

 with the feces or other body excrements. While in the encysted con- 

 dition the parasite divides b}^ binary fission into first two, then four, 

 and finally (in E. coli) eight little amoebae. If the cyst is swallowed 

 by another host, the cyst wall is dissolved and the four or eight 

 young amoebae come out to begin the active stage again. The com- 

 mon Endamoebae of man are E. gingivalis which lives in the mouth 



