^00 ANIMALS AND THEIR ENVIRONMENT 



Some leeclies feed prinuuily on small arthropods, snails and worms. 

 Others feed upon vertebrates when they are available, removing a meal 

 of blood and then falling off. A few species are completely parasitic in 

 the sense that they do not kill the host but live in continuous association 

 with it. Again, predation is associated with small invertebrates, para- 

 sitism with vertebrates. 



Bats provide a third example of this, but only the first step toward 

 parasitism has been taken. Most bats are insectivorous and feed upon 

 insects which they capture in Hight. Certain South American bats 

 have changed their food source to large mammals, and instead of killing 

 and consuming their prey they draw blood from the neck. Vampire bats 

 feed like parasites, but in their failure to remain with the host and their 

 hunting activities they are still predators. 



Commensalism and Parasitism. Commensalism and parasitism are 

 easily distinguished in theory, but in practice we know so little about 

 many organisms that we cannot be sure whether an association that 

 appears to be commensalism may not in fact be parasitism. We can only 

 say, for example, that peritrich ciliates appear to be commensals on 

 hydras, feeding upon stray bits of debris without harming the host. The 

 same is true for many of the associations found in the sea. In some 

 cases, however, the innocence of the commensal is dubious. Certain 

 marine annelids live on echinoderms, especially in the ambulacral 

 grooves of starfishes. In general these are commensals, seeking shelter on 

 the host and feeding on "leftovers" at mealtime. At least one species, 

 however, has been observed to feed on more than leftovers, poking its 

 head into the host's stomach in its enthusiasm to share the meal. The 

 evolutionary path from shelter-seeking commensalism to food-robbing 

 parasitism is not rare. 



In another type of commensalism, the commensal feeds upon ma- 

 terials shed and no longer wanted by the host. This may develop into 

 parasitism if the commensals become more aggressive, feeding first upon 

 the materials before they are shed and finally feeding on living tissues. 

 Certain kinds of mites are common in the nests of birds and mammals 

 and feed upon the shed hair, feathers and flakes of skin. This is a loose 

 type of commensalism, since the mites do not live directly on the hosts. 

 Other mites do live directly on the hosts; those feeding mostly on flaked 

 skin do little if any harm, but those feeding on feathers or hair may 

 impair the plumage or fur. These might be called commensals with 

 parasitic tendencies. Some mites have extended their diet to include the 

 living tissues of the host and thus are completely parasitic. 



Food Competition and Parasitism. The development of parasitism 

 from food competition has occurred many times in the nematodes. Both 

 free-living and parasitic nematodes are covered by a thick cuticle which 

 undoubtedly has facilitated their evolution as intestinal parasites. 

 Many species feed on fruits and vegetables in competition with other 

 herbivores. Related to these are intestinal parasites still feeding on food 

 bits, but from the security of the host's digestive tract. They may have 

 evolved from free-living forms that were inadvertently eaten. 



