BIOTIC FACTORS IN RELATION TO INDIVIDUALS 



259 



animals, exhibit monophagy, various de- 

 grees of oligophagy, and of polyphagy. It 

 remains to be discovered which of these 

 tendencies is most frequent. 



All theoretical possibilities are met iri the 

 interplay of plant and animal parasites with 

 plant and animal hosts. Plants, notably the 

 rusts, parasitize other plants. Saprolegnia 

 and many other fungi and bacteria parasi- 

 tize animals. A great variety of animals, 



Fig. 69. Caterpillar parasitized by Cordy- 

 ceps militaris, a fungus belonging to the 

 Sphaeriales. The majority of the 200-odd 

 species of Cordyceps parasitize insects. (After 

 Steinhaus. ) 



notably nematodes, parasitize plants. Most 

 major phyla of animals have representatives 

 parasitic in other animals; the echinoderms 

 do not. 



Like free-Hving animals, parasites of ne- 

 cessity meet the basic needs of ( 1 ) ecologi- 

 cal position, including food, shelter, trans- 

 port, or locomotion; and (2) reproduction 

 and dispersal. In the more advanced cases 

 of parasitism, all but reproduction are met 

 within the parasitic relationship. In sexual 

 reproduction there are numerous instances 

 in which a reduced male is parasitic on the 

 more robustly developed female of the spe- 

 cies. Parasites as a whole tend strongly to 

 specialize in reproductive activity. They 

 normally have great egg-laying ability to 

 compensate for the high mortahty at cer- 

 tain stages, and their comphcated hfe his- 

 tories abound in instances of alternating 



asexual and sexual reproduction. The great 

 development of reproductive power is made 

 necessary by the restricted size, the dis- 

 creteness, and the short life of the habitats 

 they can tolerate, and by the inefficiency 

 of their means of dispersal and means of 

 reaching their host-habitats. Each host is a 

 small, biological island, more or less com 

 pletely isolated from other host islands 

 (Kellogg, 1913). The host is furthermore a 

 hving organism, and the multiplication of 

 parasites within its tissues frequently sets 

 up immunity reactions such that the para- 

 site population, to survive, must find an- 

 other host whose tissues are unmodified. 



These same characteristics increase the 

 difficulties in establishing contact between 



Fig. 70. Association of the sexes in the para- 

 sitic trematode, Schistosomiim haematobium, 

 which lives in the larger blood vessels of man, 

 reaching a length (in the male) of about 15 

 mm.; S, sucker. (After Hesse and Doflein.) 



the sexes. Various devices have been 

 evolved that aid in securing cross fertiliza- 

 tion. Many parasites are hermaphroditic, 

 and a part of the elaborate development ol 

 the accessory sex organs in trematodes 

 seems best understood as adaptations that 

 help avoid self-fertihzation (Borradaile and 

 Potts, 1935). A spectacular adjustment en- 

 suring cross fertilization is that of Diplo- 

 zoan, a trematode parasitic on the gills of 

 minnows, in which two hermaphroditic ani- 



