134 ZOOLOGY 



this second host and the eggs or young embryos escape into 

 the water with the excreta and from there are taken up by 

 other trout whose destruction is again wrought by the tissue- 

 infesting larvae. This passage from one host to another prob- 

 ably arose, and is helped, by the carnivorous habit among 

 animals. 



The parasites are almost exclusively invertebrates. The 

 worms and arthropods furnish the most numerous representa- 

 tives. The gregarines, and some other Protozoa, are internal 

 parasites, often being parasitic within the cells . There are only 

 a few parasitic vertebrates, and these are transient. They be- 

 long to the lower fishes (lamprey. Fig. 64) . 



Parasitism is a very successful adaptation to a much limited 

 environment in which the organism has bartered its original 

 powers for a life of comparative ease. The only necessity 

 still resting upon it is in the matter of reproduction, and the 

 success with which this needful function is accomplished shows 

 us that the parasite must be considered well adapted to its 

 conditions, notwithstanding its degeneracy. Its chief hazards 

 are met in the passage from host to host and these are over- 

 come by the carnivorous and omnivorous habits of hosts and 

 the extraordinary powers of multiplication on the part of the 

 parasites. 



165^ Practical Exercises. — Enumerate all the parasites, transient and per- 

 manent, known to infest man, and find to what groups of animals they belong. 

 Report on the habits of the principal parasites on man: as tape- worm, trichina, 

 hook-worm, etc. What other hosts are demanded to complete the life cycle? 

 What are the principal sanitary conclusions to be reached? Examine the mouth- 

 parts of the mosquito (see Fig. 65). To what kind of feeding are they adapted? 



166. Habits and Instincts in Relation to Adaptation. — In 



the study of adaptations there is constant danger lest we 

 come to consider that structures alone are adaptive. In reality, 

 adaptation in the manner of doing things is quite, as important 

 as in the structure of the organs by which work is done. When 

 even the simplest organisms are acted on by an external stimulus 

 they respond to it in some way. This response may be either 

 advantageous or disadvantageous to the organism. If unfav- 

 orable, the result may be disastrous. If favorable later repe- 



