GENERAL CONSIDERATIONS 7 



While parasitism will be dealt with by others in this volume (see 

 Becker, infra, Chapter XVII; and Kirby, infra, Chapters XIX, XX), 

 I will speak here merely of one or two types of adaptations which have 

 arisen as a result of this mode of life. Ectoparasites and endoparasites 

 have developed somewhat differently, as a result of their different modes 

 of life and different needs. In common, they all possess the first great 

 need of all parasites, viz. reproductive power, thus obeying a first law 

 of nature to the effect that the number of offspring produced should vary 

 according to the difficulties encountered in their youth and during de- 

 velopment to maturity. 



Adaptations of ectoparasitic types are mainly morphological, and here 

 some striking structures may be developed, as a few illustrations will 

 show. All may have a special feeding advantage by being transported 

 from place to place; or when attached to gills or other structures of their 

 hosts, they are continuously bathed by food-bearing currents of water. At- 

 tached forms live on algae, exoskeletons and appendages of arthropods, 

 shells of molluscs, or gill filaments and gill bars of all kinds of water- 

 dwelling animals. Thus we find species of Zoothamnium or of Lagen- 

 ophrys on the legs of Gammarus, while species of Spirochona or Den- 

 drocometes are on the gill filaments of the same hosts, or on Asellus. 

 The suctorian Trichophrya adheres like a saddle to the gill bars of Salpa, 

 while the vorticellid Ellobiophrya encircles a gill filament of the lamelli- 

 branch Donax vittatus by the union of two branch outgrowths of a more 

 common adhesion disc (see Kirby, Fig. XXX). The latter is almost 

 always provided with hooks or suckers, or both, to form the "scopula," 

 as in species of Trichodina or Cyclochaeta, parasites on Hydra. A special 

 thigmotactic reaction appears to keep the ciliate Kerona pediculus on the 

 surface of Hydra fusca. Such forms seem to have no ill effects on their 

 hosts and scarcely qualify as parasites. Schroder calls them "epibionts." 

 Real ectoparasites are rare, as a matter of fact, and they are little differ- 

 ent from free water-dwelling forms in structure. They do occur, how- 

 ever, especially on fish hosts. The flagellate Costia necathrix increases to 

 such numbers that normal functions are impeded, and young fish are 

 frequently killed. Ichthyophthirius midtifiliis bores into fish skin and 

 brings about distributed ulcerations which may be fatal. 



Endoparasites may be more destructive, and, while they are relatively 

 simple morphologically, they may be highly differentiated physiologically. 



