96 UNDERHILL : 



from the unicellular to the vertebrate, and, though we may 

 not make a sharp distinction between predaceous and para- 

 sitic animals, in view of the degrading influence of the para- 

 sitic habit we should clearly define the difference between the 

 simplicity of degeneration and the simplicity of primitiveness. 

 In the development of a primitively simple animal the young 

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

 simple ancestors. In the degenerate animal, on the other 

 hand, the ancestors are often more complex, and the young 

 stages of a higher grade than the stage of the adult, and the 

 adoption of any mode of life which withdraws from the activ- 

 ities and shirks in the pursuit of food seems to bring about 

 this condition of degradation. Of this we have a remark- 

 able example outside of the realm of parasitism in the Tun- 

 icata. These aberrant animals in the stage of the free-swim- 

 ming larva have a chordal axis which in nearly all of the dif- 

 ferent species becomes entirely lost before they reach matur- 

 ity. 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 known 

 as "sea squirts," mere, attached, plant-like sacks, 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 mod- 

 ified into claws or hooks for clasping the hairs of the host, or 

 may more or less completely disappear 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 parasite is nourished need scarcely any digestion, the 

 digestive organs become simplified or may be quite lost, and 

 the absorbtion of nutriment take place entirely through the 

 body integument, as in the case of some of the worms which 



