^1(3 ANIMALS AND THEIR ENVIRONMENT 



Both undoubtedly weaken the host but their actual damage is difficult 

 to estimate. Both can be controlled. Sleeping sickness, caused by a 

 blood sajMozoite, is the scourge ot much of Africa. These five diseases 

 are probably the most important, if both the seriousness of the disease 

 and the nimiber of people affected are taken into consideration. The 

 extent to which tour of these parasites are distributed in the world is 

 shown on the accompanying maps (Fig. 39.15). Entamoeba histolytica 

 is virtually world-wide, but is a serious problem only in the tropics. 



366. Adaptations to Parasitism 



Adaptations that are common among parasitic animals include the 

 development of devices for attachment and of methods of transmission, 

 and simplification or loss of sensory, locomotor and digestive structures. 

 These adaptations are found in other organisms, of course, and none 

 of them is found in all parasites. 



Means for Attachment. Devices for attachment are especially 

 common among ectoparasites and intestinal parasites. The suckers of 

 trematodes (Fig. 11.11) and leeches (Fig. 15.1) are obvious examples. 

 The burrowing habit of some of the skin mites is a less obvious way of 

 solving the attachment problem. Most of the fleas and lice have legs 

 and claws adapted for gripping hair or feathers. In the human crab 

 louse, for example (Fig. 39.3), the second and third pairs of legs are 

 chelate in such a way that when the claw closes against the "hand" 

 a hole is left that is slightly smaller than the diameter of a pubic hair. 

 This enables the louse to grip pubic hairs tightly without cutting them 

 through. These lice are limited to the pubic region primarily because 

 the head and body hair is too fine to be gripped, but men with lux- 

 uriant coarse body hair can be infested from head to toe. The head 

 and body lice (Fig. 39.3) have more delicate claws. 



The ventral sucker of intestinal trematodes is used for attachment 

 inside the body just as the posterior sucker of their ectoparasitic rela- 

 tives is used on the outside. The suckers or hooks of tapeworms, the 

 spiny heads of acanthocephalans, and the ventral concavity of Giardia 

 have already been described. Hookworms are securely attached by the 

 mouthful of intestinal wall through which they suck blood (Fig. 39.9). 

 Prominent among intestinal parasites that are not attached are Ascaris 

 and its relatives. These continually crawl "upstream" as a means of 

 staying in the host (they occasionally crawl too far and come out the 

 mouth or nose). 



Means for Iransmhs'ion. Two problems are involved in the trans- 

 fer of the parasite from one host to another: the development of stages 

 in the life cycle that can survive crossing the ecologic desert that lies 

 between hosts, and the production of sufficient numbers of such stages 

 to enhance the chance of locating a new host. The first problem is 

 associated with the survival of the individual, the second with the sur- 

 vival of the species. 



Organisms that are only partially modified as parasites, for ex- 

 ample leeches and mosquitoes, have no difficulty getting from host to 



