PARASITISM, ETC., ON THE SEA-SHORE 141 



among Nematodes " scarcely a stage that may not be 

 parasitic ; the eggs, the larvae, the adults are all in some cases 

 free, in others parasitic, and in many cases first the one and 

 then the other " (Shipley, 1896. See also Borradaile, 1918). 

 Parasitism exerts a marked effect on nearly all the animals 

 which practise it, whether they are ectoparasites or endo- 

 parasites. There is frequently a loss of activity, of appen- 

 dages, and of sense organs on the part of the parasite ; the 

 form of the parasite is more or less modified and adapted to 

 that of the host, or to suit that part of the host where it 

 lodges. Organs such as hooks, spines, suckers, and claspers 

 are developed to facilitate the adhesion of the adult, or to 

 assist the entry of the larval parasite into the host's body. 

 So profound is the modification in some instances that the 

 place of the parasite in classification can only be determined 

 by reference to its development. Hermaphroditism and 

 great fertility are characters very generally exhibited by 

 endoparasites. The effect on the host is sometimes severe 

 and may cause death. Malnutrition and growth abnor- 

 malities frequently resuh from the presence of a parasite, 

 and in extreme cases the host's sex-organs may be rendered 

 sterile. Some parasites, however, do not cause recognisable 

 disturbance in their host or do so only when present in 

 unusual numbers. Others, such as tapeworms, have been 

 shown to produce substances which act as toxins upon their 

 host and have effects analogous to those produced by 

 pathogenic bacteria. Generally speaking, a balance is main- 

 tained between host and parasite which tends to Hmit the 

 amount of injury, since the death of the host would, in 

 many cases, entail the death of the parasite also. The 

 number of parasites affecting a particular host varies very 

 considerably ; one individual may at times harbour thousands 

 while in other cases only a few are normally present. Atten- 

 tion has been drawn to the fact that many endoparasites 

 derive their sustenance from the tissue fluids of their host 

 or from the liquid nutriment in the latter's gut. Such forms, 

 though they may be associated with land animals, are 

 aquatic rather than terrestrial in their mode of life. 



