ANIMAL PARASITISM 



747 



class Hexapoda or Insecta contains, besides several hundreds of thou- 

 sands of free-living insects, the parasitic fleas, lice, and bedbugs; 

 the class Arachnida, characteristically free-living, contains the para- 

 sitic ticks and mites, and the class Crustacea, though mostly free-liv- 

 ing, includes a number of species parasitic on fishes and other aquatic 

 animals. While most of the parasitic arthropods are ectoparasites, 

 there are also a few endoparasitic species. For example the horse 

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

 ach of horses; long wormlike arachnids known as Linguatulids or 

 tongue worms are found in the intestines of some reptiles and mam- 

 mals; and Sacculina (Fig. 404), a crustacean, parasitic on crabs and 

 lobsters, sends rootlike outgro\vths all through the body of its host, 

 although the saclike body remains on the outside. 



Some Representative Parasites 

 Protozoa. — The very small amoebalike protozoans of the genus 

 Endamoeba are examples of parasites only slightly modified for 

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



Fig. 391. — Amoeba histolytica, one of the important protozoan parasites. It is 

 the causal a.erent of amoebic dysentery. A, Stained vegetative amoeba ; B, cyst 

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

 karj-osome ; r.b.c, ingested red blood corpuscles; chr.b., chromatoid body. (Re- 

 printed by permission from Introduction to Human Parasitology by Chandler, pub- 

 lished by John Wiley and Sons, Inc., after Dobell.) 



V\ 



i 



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

 podia are shorter and move more slowlj'"; these active forms finally 

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

 In this encysted condition Endamoeba is passed from the host 

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



