EFFECTS ON PARASITES 153 



lice are exceptional in feeding on feathers which they seize with 

 biting mouthparts. Often ectoparasites also possess some means 

 of attachment, such as the posterior sucker of leeches and the 

 well-developed claws of lice. Gut-living endoparasites also often 

 possess means of attachment, such as the hooks and suckers of 

 tapeworms and the jaws and sucking mouth of the hookworm ; 

 most nematodes, however, have none. Many parasites possess at 

 some stage of their life-history considerable powers of locomotion, 

 as the frequent occurrence of the sentence ' bores its way into the 

 tissues ' in the accounts of life-histories shows. Once in place they 

 may not move, but they have seldom lost any organs of locomo- 

 tion other than those specially developed for the use of the 

 larvae. The flatworms show a progressive loss of cilia. The uniform 

 environment in which many parasites live, at least as adults, 

 presumably means that they receive few stimuli, and one might 

 therefore expect there to be few sense organs. This is found to be 

 true, but at the level of body-organisation of the three major 

 parasitic groups there would be few sense organs anyway. The 

 abihty of the Guinea worm to get its head to the right place 

 at the right time suggests a co-ordinated response of a high order. 

 Most parasites are well supplied with food, and often this is 

 of a simple form ; in the small intestine and in the blood stream 

 there are hexoses and amino-acids in place of the polysaccharides 

 and proteins which are all that most free-living forms can get. 

 Parasites in such situations therefore need few if any digestive 

 enzymes, and can begin synthesis or oxidation straight away ; 

 it is tempting to connect the absence of the gut from the cestodes 

 with this, but it must be remembered that the free-living 

 Platyhelminthes show a progressive degeneration of the gut, 

 and that some of the turbellarians have it replaced by a solid 

 mass of cells. It seems, therefore, that parasitism need not have 

 caused the absence of the gut from the tapeworms, but that the 

 parasitic mode of life allowed an innate tendency to go to an 

 extreme. Most nematodes have a well-developed gut wherever 

 they live. Parasites such as the hookworms, which eat the tissues, 

 are in no different case from any animals to whom food comes 

 easily. It is possibly the large food supply that has induced the 

 large reproductive capacity of parasites, for it is a general 

 observation that much food means many eggs. Pelagic fish, the 

 queen honey bee, and the barndoor hen are non-parasitic 

 examples. Parasites have as much need to excrete as have any 

 M.z. — 6 



