4 PARASITES OF THE DOMESTIC ANIMALS 



clasping the hairs of the host, or it may almost if not completely dis- 

 appear and be replaced by such organs of fixation as sucking-disks. As 

 the contents of the alimentary canal or tissue fluids of the host upon 

 which the verminous parasite is nourished need scarcely any digestion, 

 the digestive organs become simplified or may be quite lost, the absorb- 

 tion of nutriment in the latter case taking place entirely through the 

 body integument, as in some of the worms which infest the intestines of 

 man and other animals. The degree of decadence will depend upon the 

 degree of dependence upon the host: In this latter respect the parasitism 

 may be optional, as in the case of the mosquito, which may live upon the 

 juices of plants but prefers a meal of warm blood, or it may be obligate, 

 depending upon another for its means of subsistence, though such 

 obligate parasites as the biting flies, fleas, and bedbugs may also live 

 free and only occasionly visit their hosts, a forin of parasitism which 

 may be accompanied by Httle modification of the adaptability to a free 

 life. 



In the event of the parasite becoming progressively degraded into one 

 which not only seeks its host for food, but has become dependent upon 

 it for both its nutrition and place of abode, all of the above mentioned 

 phenomena of adaptation become more conspicuous. There is furnished 

 a very good example of such a transformation in the sheep tick (Melo- 

 phagus ovinus), not a true tick, however, but a fly which, originally an 

 occasional visitor, has, like the louse, taken permanent abode upon its 

 host. No longer taking the aerial flight of its discarded free life, this 

 fly has become wingless, and, furthermore, is enabled to pass its .entire 

 life cycle upon the body of the host animal by a remarkable method of 

 reproduction involving the retention of the eggs in the oviducts until 

 development has passed through the larval stage. It is not until ready to 

 pass into the stage of the pupa that the larvae are extruded, the pupal 

 case then being attached to the individual wool fibers. From this case 

 the young insect, on becoming sufficiently developed, makes its escape 

 and proceeds to feed and grow, thus rounding out a complete parasitic 

 cycle. 



While the easy life of the parasite tends to degeneration, the perpetua- 

 tion of the species becomes more precarious, and the organs of reproduc- 

 tion undergo a marked development. If a host animal dies most of its 

 parasites, especially those existing in the interior of its body, die with 

 it, and, were it not that the eggs find lodgnient in a new host, the parasitic 

 species would in a short time become extinct. The transmission of but 

 few of these eggs is successfully accomplished, and in compensation they 

 must be produced in enormous numbers, well protected from the many 

 elements of destruction which they encounter. The mode of reproduc- 

 tion is one of the principal factors determining the conditions of par- 

 asitism, and, while the above modifications pertain more to those 



