PARASITISM AND DEGENERATION 



363 



leiLcopterus) of the Mississippi Valley is a parasitic fungus (Sporo- 

 trichum glob idijer urn). In the autumn, house flies may often 

 be seen dead against a windowpane surrounded by a delicate 



ring or halo of white. 



This ring is 



composed of spores of the fungus, Em- 

 pnsa aphidis, which has grown through 

 all the tissues of tlie flv wliile alive, 

 finally resulting in its death. The 

 spores are thrown off from tiny fruit- 

 ing hypha) of the fungus which have 

 grown out through the body wall of 

 the insect. And they serve to inocu- 

 late other flies that may come near. 



Just as in animals, so in })lants; 

 parasitic kinds, especially among the 

 higher groups as the flowering j^lants, 

 often show marked degeneration. Leaves 

 may be reduced to mere scales, roots are lost, and the water- 

 conducting tissues greatly reduced. This degeneration in plants 

 naturally affects primarily those parts which in the normal 

 plant are devoted to the gathering and elaboration of inor- 

 ganic food materials, namely, the leaves and stems and roots. 



Fig. 222.— The itch mite, 

 Sarcoptcs acabeu 



Fig. 223. — The fungus, Cordiceps, growing on a caterpillar. (Natural size.) 



The flowers or reproductive organs usually retain, in parasites, 

 all of their high development. 



While parasitism is the principal cause of degeneration of 

 animals, other causes may be also concerned. Fixed animals 

 or animals leading inactive or sedentary lives, also become 

 degenerate, even when no parasitism is concerned. The tuni- 

 cata or s'^a squirts (Fig. 224) are animals whose simplicity of 

 structure is due to degeneration from the acquisition of a 

 sedentary habit of life. 



The young or larval tunicate is a free-swinuning active tad- 

 polelikc creature with organs much like those of the adult of 



