PARASITISM 819 



cercaria of trematodes, many have various organs including eyes in the 

 free-living stage but these are absent from the parasitic stage. 



The evolutionary reduction of organs is called simplification or 

 degeneration. It would be a mistake, however, to consider that parasitic 

 organisms are "degenerate" because they have some degenerate organs. 

 Degenerate organisms are ineffective, inefficient individuals. Parasites 

 are both efficient and effective. The degeneration— simplification or 

 loss— of some of their organs is balanced by other adaptations with which 

 they exploit the parasitic way of life. Sucking lice, for example, are 

 abundant wherever mammals are found, and in spite of their weak 

 bodies and near or total blindness they live with remarkable security, 

 having longer lives and requiring fewer offspring for perpetuation of 

 the species than many free-living insects of their size. 



367. Host Specificity 



Many parasites can infect a variety of animals. The common tick 

 (Fig. 39.5) will feed on almost any mammal, and a single acantho- 

 cephalan species may be found in the intestines of birds belonging to 

 several different orders. Most parasites, however, are more restricted 

 and infect only a group of species that are closely related. One genus 

 of tapeworms is foiuid only in carnivores, another only in rodents and 

 a third only in marsupials. Some parasites are still more restricted and 

 can infect only one host species or possibly a few species of the same 

 genus. This extreme host specificity is common in malarial parasites 

 (those of man will not infect any other animal), sucking lice (the crab 

 louse can live on the gorilla but the head and body lice live only on 

 man), and nematodes (the human Ascaris can live in other mammals 

 but will not reproduce there), and it is not rare in other groups such as 

 fleas and tapeworms. 



Where host specificity is extreme the parasites may have been as- 

 sociated with their hosts for a considerable period of geologic history, 

 and as the host evolved into a number of species the parasites evolved 

 with them. In such cases the taxonomic arrangement of the hosts and 

 the parasites often shows similar or identical patterns. This phe- 

 nomenon has been used as a means of settling certain taxonomic 

 problems. In the last century, for example, it was observed that the 

 llamas of the South American Andes were similar to the camels of 

 northern Africa and central Asia, but the geographic distance between 

 the two groups was considered to be a barrier to placing them in the 

 same family. When their lice were studied it was discovered that they 

 also were similar to each other and different from other lice. On the 

 strength of this concordance the llamas and camels were grouped in 

 the family Camelidae and the lice were grouped in the genus Micro- 

 thoracius. This decision was shown to be correct later when an 

 abundance of fossil camels was found in North America. Today, in 

 fact, it is believed that the group together with its lice arose in North 

 America and spread to both South America and Asia before becoming 

 extinct on this continent. 



