PARASITES AND THEIR DEVELOPMENT. 195 



host, and may thus have become transformed into organs of 

 nourishment. By-and-by the sedentary life, with its advan- 

 tages in the way of cheap living and easy existence for the 

 sacculina, would become a fixed habit. The sacculinae, which 

 acquired this habit, together with their descendants, would 

 flourish and increase in numbers owing to the advantage 

 gained by them in that " struggle for existence " in which 

 sacculinae and their highest animal-neighbours are forced, one 

 and all, to take part. And as the wholly free sacculinae 

 became transformed into higher forms of life, or became 

 extinct, their rooted and parasitic brethren may be regarded 

 as having gradually degenerated. A process of physiological 

 backsliding invariably takes place in such cases. This retro- 

 gression would be manifested in the sacculinse by the casting 

 off of structures which were no longer of use to a fixed and 

 rooted being, the degeneration and disappearance of struc- 

 .tures not required in the animal economy, taking place in 

 virtue of the well-known law of the "use and disuse" of 

 organs. The legs would thus become gradually diminished, 

 and would finally disappear altogether. Internal organs, and 

 parts useful to the free-swimming animal, would become use- 

 less as the creature became more and more dependent on its 

 host. Finally the sac-like organism would be evolved as the 

 result of its parasitic habits ; and the degeneracy which marks 

 the slavishly dependent mind in higher life is thus viewed 

 as also destroying the independence and as warping and dis- 

 torting the character which once marked the free and active 

 creature of lower grade. Thus we may understand by the 

 study of life-histories such as those of the sacculina and its 

 comrades, how parasitism is induced, and how a change of 

 life and habits of such sweeping character, converting an 

 active being into a sedentary and degraded animal, becomes 

 established through the slow but sure effects of habit, use, 

 and wont, perpetuated through many generations. 



Perhaps the most inveterate and dreaded enemy which 

 man has to encounter in the ranks of parasites is the little 

 Trichina (Fig. 23), which has, on more than one occasion, 



