INTRODUCTION 3 



Whether this relationship between different species is of reciprocal 

 advantage or of benefit to but one, neither of the symbionts lives upon 

 or at the expense of its co-symbiont, and neither has entirely renounced 

 its independence. In true parasitism the invading animal lives upon the 

 tissues of its host, deprives it of a portion of its nourishment; or is in 

 other ways injurious to it. There are many examples of this form of 

 symbiosis, and students of animal life are familiar with the conditions 

 that seem always to attend it, such as the degenerative and adaptive 

 modifications occurring in the parasite. 



It is the common habit of many animals, however, to prey upon the 

 bodies of other animals, and we should distinguish, so far as we may, 

 between those which are predatory and those which are parasitic. The 

 former are free and exercise their powers of sense and cimning in snaring 

 or chasing their prey, while the latter, in fully acquired parasitism, live 

 on or in the bodies of their victims, often burrowing into and consuming 

 the body tissues, leading a lazy, beggarly existence in which all of the 

 faculties of special sense and prowess, so highly developed in predatory 

 animals, become degenerate and atrophied. 



Parasitism is foimd throughout the range of animal life from the 

 unicellular to the vertebrate, and, though a sharp distinction between 

 predaceous and parasitic animals may not be made, in view of the de- 

 grading influence of the parasitic habit, the difference between the 

 simplicity of degeneration and the simplicity of primitiveness should be 

 clearly defined. In the development of a primitively simple animal the 

 young stages are more simple than in the adult and it has only simple 

 ancestors. In the degenerate animal, on the other hand, the ancestors are 

 often more complex and the young stages are of a higher grade than the 

 stage of the adult. The adoption of any mode of life which withdraws 

 from the activities necessary to survival in a free existence seems to 

 bring about this condition of degradation. Of this we have a remarkable 

 example outside of the realm of parasitism in the Tunicata. These 

 aberrant animals, in the stage of the free-swimming larva, have a chordal 

 axis which in nearly all of the different species becomes entirely lost 

 before they reach inaturity. After passing the "tadpole" stage there 

 follows an extreme specialization to the fixed habit which most tunicates 

 retain throughout their adult life, becoming what are commonly known 

 as sea squirts, mere attached, plant-like sacs, emitting a jet of water 

 when disturbed, and from which all chordate features have been entirely 

 lost. 



The degenerative changes which a parasite undergoes concern mostly 

 the nervous system, the organs of locomotion, and those of nutrition, the 

 nervous system becoming reduced to the most indispensable portion, 

 while of the sense-organs nothing may be left except those of touch. The 

 locomotor apparatus may become modified into claws or hooks for 



