EFFECT OF PARASITISM ON THE PARASITE 4I 



itself. It may have no alternative as a short term poHcy, but in the long 

 run such a procedure may prove fatal to the race. 



Apart from the modifications connected with reproduction, there 

 are certain morphological features which recur persistently in para- 

 sites. Organs of locomotion are partially or totally lost. Intestinal 

 worms and leeches have no ambulatory processes. Parasitic insects 

 which live on the bodies of birds, such as feather hce, fleas and bugs 

 and certain flies, are wingless or have mere vestiges of wings which 

 for the purpose of flight are useless. On the other hand, they have 

 developed very varied and efficient organs of attachment, such as 

 hooks, suckers, anchor-like protuberances and prehensile lips — "for 

 their strength is to sit still." In the case of ticks, fleas and some 

 fly-larvae the mouth-parts, which are embedded in the host's flesh, 

 are armed with re-curved spines (see Plate XII). Leeches, on the 

 other hand, have cup-like suckers at both ends of the body, some 

 flukes chng grimly from the rear only, whereas lice hang on by 

 their claws. It is quite obvious that once a parasite has reached a 

 suitable host it must make every effort to remain with it. To be 

 sneezed out of the nasal cavity of a duck or blown out of an elephant's 

 trunk are very great dangers which leeches must guard against. Birds 

 with the "gapes" are racked by coughing — a sort of recurrent earth- 

 quake for the worms in their throats — and it is, therefore, scarcely 

 surprising that these nematodes live with their anterior ends embedded 

 in the mucous membrane of the bird's trachea. In a way, parasites are 

 caught between the devil and the deep sea, for often organs of loco- 

 motion would be extremely useful for finding their host, but a distinct 

 disadvantage once they have achieved this object. The parasitic fly 

 Carnus hemapterus, directly the bird host is found, quickly creeps 

 between the feathers, but first casts off its own wings. This is an un- 

 usual case. Generafly parasites lose their own organs of locomotion and 

 employ other transport animals (see p. i8 and tail-piece of Chapter 8) 

 in order to reach their host. 



Many internal parasites absorb food through the surface of the body. 

 There is a tendency to lose their mouths and part, or all, of the digestive 

 organs. This modification is found in some Protozoa as well as in 

 worms and various other parasites. In ticks and leeches, which 

 frequently have to endure long fasts between their meals, portions of 

 the ahmentary canal are extended in the form of pouches and branches 

 in which the blood is stored and from which it can be absorbed slowly. 



